Abstract:
A method is provided for authenticating characteristics of electrical energy. The method comprises acquiring a key, acquiring an amount of electrical energy, and generating a digital signature based on the amount and the key. The method further comprises generating a certificate comprising the signature and the amount.

Description:
TECHNICAL FIELD 
     The present invention relates to a green energy generation apparatus, a mobile device, an electric storage apparatus, and a management method of green energy information. 
     BACKGROUND ART 
     The global environment has been worsening seriously in recent years, and each country has been taking environmental measures. Under such circumstances, an energy generation method using renewable energy, such as solar energy, wind energy, geothermal energy (hereinafter, referred to as the renewable energy generation method), and an energy generation method using environmentally friendly resources, such as biomass energy, fuel cell, and the like (hereinafter, referred to as the environmentally friendly energy generation method) are drawing a great deal of attention. At present, a thermal energy generation method using fossil fuels, such as oil, coal, and the like, a nuclear energy generation method using atomic energy, and a hydroelectric energy generation method using flowing water are mainly used. 
     The nuclear energy generation method is an environmentally friendly energy generation method, but has difficulties and risks in the safety of managing atomic fuels. The hydroelectric energy generation method is also environmentally friendly; however, only areas with geographical conditions suited for constructing dams, or the like, can utilize this method. The thermal energy generation method is facing problems, such as the drain of fossil fuels, emissions of CO 2 , NO x , and the like, generated from burning fossil fuels. In light of the circumstances, replacing the thermal energy generation method with the renewable energy generation method or the environmentally friendly energy generation method is under discussion. 
     In recent years, individual electric energy consumers have become more conscious of global environmental issues, and there is a movement to use an energy generator to implement the renewable energy generation method or the environmentally friendly energy generation method (hereinafter, referred to as a green energy generation apparatus) in a household. Further, in order to resolve problems of instability in the supply of energy, which is a weak point of the renewable energy generation method, there is also a movement to use an electric storage apparatus in a household along with the energy generator implementing the renewable energy generation method. In other words, individual electric energy consumers are obtaining the green energy generation apparatus and the electric storage apparatus for their households at a rapid pace. In the near future, a greater number of individual electric energy consumers will generate electric energy in their households, and cover their own consumption with the electric energy they generate themselves. 
     At present, electric utility companies are buying the surplus electricity that individual electric energy consumers generate but cannot consume themselves. However, for the electric utility companies, the electricity generated by individual electric energy consumers is an unstable energy source, the capacity and timing of supply being unpredictable. Such electricity is not attractive for the electric utility companies, so the prices of electricity sold to electric utility companies from individual electric energy consumers tend to be low. The amount of electricity produced by each of the individual electric energy consumers is extremely small compared to the capacity currently traded in the energy trading market; therefore, individual electric energy consumers cannot sell the electricity in the energy trading market in the present situation. 
     Further, the existing energy trading market comprises limited players. In fact, it is practically impossible for individual electric energy consumers to trade in the energy trading market. Regarding electricity trading, Japanese Unexamined Patent Application Publication No. 2008-225755 discloses a method for predicting precisely the amount of electricity to sell and to buy based on past trading history in the energy trading market. Regarding the structure of the current electricity trading, information distributed from the Japan Electric Power Exchange, the Chicago Mercantile Exchange (CME), or the like, must be useful. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2008-225755 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Under the circumstances, it is expected that a structure effectively utilizing the electricity generated by individual electric energy consumers using green energy generation apparatuses (hereinafter, referred to as green electricity) will be realized. It is also expected that an energy trading market of a small size, which individual electric energy consumers can join, will be realized. For example, an energy community, where individual electric energy consumers can accommodate each other with electricity by trading their surplus electricity, is desired for each local area. In order to realize this, however, there are many issues to be resolved, such as how each individual electric energy consumer can be identified, how arbitrage prices of electric energy should be decided, and the like. A structure that resolves these issues is also expected. 
     In light of the foregoing, it is desirable to provide a green energy generation apparatus, a mobile device, an electric storage apparatus, and a management method of green energy information, which are novel and improved, and which are capable of generating a certificate to authenticate green energy. 
     Solution to Problem 
     Accordingly, there is provided a method for authenticating characteristics of electrical energy. The method comprises acquiring a key, acquiring an amount of electrical energy, and generating a digital signature based on the amount and the key. The method further comprises generating a certificate comprising the signature and the amount. 
     In another aspect, there is provided an apparatus for authenticating characteristics of electrical energy. The apparatus comprises an energy generator and a processor configured to acquire a key, acquire an amount of electrical energy generated by the energy generator, and generate a digital signature based on the amount and the key. 
     Consistent with the description above, it is possible to build a structure to generate a certificate to authenticate green energy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an explanatory diagram for illustrating a configuration of a green energy generation system consistent with the first embodiment of the present invention; 
         FIG. 2  is an explanatory diagram for illustrating a functional configuration of a green energy generation apparatus according to the present embodiment; 
         FIG. 3  is an explanatory diagram for illustrating a functional configuration of a certificate of electric-generation issuance unit according to the present embodiment; 
         FIG. 4  is an explanatory diagram for illustrating a functional configuration of an electric storage apparatus according to the present embodiment; 
         FIG. 5  is an explanatory diagram for illustrating a functional configuration of a certificate of electric-storage issuance unit according to the present embodiment; 
         FIG. 6  is an explanatory diagram for illustrating a functional configuration of an interface apparatus according to the present embodiment; 
         FIG. 7  is an explanatory diagram for illustrating a configuration of a green energy generation system consistent with a second embodiment of the present invention; 
         FIG. 8  is an explanatory diagram for illustrating a functional configuration of an electric storage apparatus according to the present embodiment; 
         FIG. 9  is an explanatory diagram for illustrating a functional configuration of a certificate of remaining issuance unit according to the present embodiment; 
         FIG. 10  is an explanatory diagram for illustrating a functional configuration of a certificate of space issuance unit according to the present embodiment; 
         FIG. 11  is an explanatory diagram for illustrating a functional configuration of an interface apparatus according to the present embodiment; 
         FIG. 12  is an explanatory diagram for illustrating a functional configuration of a certificate of consumption issuance unit according to the present embodiment; 
         FIG. 13  is an explanatory diagram for illustrating a functional configuration of an energy trade server according to the present embodiment; 
         FIG. 14  is an explanatory diagram for illustrating a configuration of a green energy generation system according to the third embodiment of the present invention; 
         FIG. 15  is an explanatory diagram for illustrating a functional configuration of a green energy generation apparatus according to the present embodiment; 
         FIG. 16  is an explanatory diagram for illustrating a functional configuration of a certificate of electric-generation issuance unit according to the present embodiment; 
         FIG. 17  is an explanatory diagram for illustrating a functional configuration of an electric storage apparatus according to the present embodiment; 
         FIG. 18  is an explanatory diagram for illustrating a functional configuration of an interface apparatus according to the present embodiment; 
         FIG. 19  is an explanatory diagram for illustrating a flow of operation for transmitting electric energy according to the present embodiment; 
         FIG. 20  is an explanatory diagram for illustrating a flow of operation for transmitting electric energy according to the present embodiment; 
         FIG. 21  is an explanatory diagram for illustrating an example of a hardware configuration capable of realizing functions of various types of certificate issuance units, interface apparatuses, mobile devices, energy trade servers, or the like. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     (Flow of Explanation) 
     The flow of explanation described below regarding the embodiment will be explained in brief. At first, an explanation will be given regarding a configuration of a home energy network  1  according to the first embodiment with reference to  FIG. 1 . Then, with reference to  FIG. 2  and  FIG. 3 , an explanation will be given regarding a green energy generation apparatus  11  according to the present embodiment. With reference to  FIG. 4  and  FIG. 5 , an explanation will be given regarding a configuration of an electric storage apparatus  12  according to the present embodiment. With reference  FIG. 6 , an explanation will be given regarding a configuration of an interface apparatus  13  according to the present embodiment. 
     Next, with reference to  FIG. 7 , an explanation will be given regarding a configuration of an energy trade system according to the second embodiment of the present invention. With reference to  FIG. 8  through  FIG. 10 , an explanation will be given regarding a configuration of an electric storage apparatus  12  according to the present embodiment. Next, with reference to  FIG. 11  and  FIG. 12 , an explanation will be given regarding a configuration of an interface apparatus  13  according to the present embodiment. Then, with reference to  FIG. 13 , an explanation will be given regarding an energy trade server  5  according to the present embodiment. 
     With reference to  FIG. 14 , an explanation will be given regarding configurations of a mobile device  6  and an energy reception apparatus  7  according to the third embodiment of the present invention. With reference to  FIG. 15  and  FIG. 16 , an explanation will be given regarding a green energy generation apparatus  61  according to the present embodiment. With reference to  FIG. 17 , an explanation will be given regarding a configuration of an electric storage apparatus  62  according to the present embodiment. With reference to  FIG. 18 , an explanation will be given regarding a configuration of an interface apparatus  63  according to the present embodiment. Next, with reference to  FIG. 19  and  FIG. 20 , an explanation will be given regarding an operational method for transmitting electric energy according to the present embodiment. 
     Subsequently, an explanation will be given regarding an example of a hardware configuration capable of realizing functions of an interface apparatus, a mobile device, an energy trade server, or the like. Lastly, a technical philosophy of the embodiments will be described and a brief explanation will be given regarding functional effects derived from the technical philosophy. 
     (Description Items) 
     1: First Embodiment (Issuing a Green Certificate) 
     1-1: Configuration of Home Energy Network  1   
     1-2: Configuration of Green Energy Generation Apparatus  11   
     1-3: Configuration of Electric Storage Apparatus  12   
     1-4: Configuration of Interface Apparatus  13   
     2: Second Embodiment (Structure of Energy Trading Market for Individual Consumers) 
     2-1: Configuration of Energy Trade System 
     2-2: Configuration of Electric Storage Apparatus  12   
     2-3: Configuration of Interface Apparatus  13   
     2-4: Configuration of Energy Trade Server  5   
     3: Third Embodiment (Services based on a Green Certificate) 
     3-1: Configuration of Mobile Device  6   
     3-2: Configuration of Green Energy Generation Apparatus  61   
     3-3: Configuration of Electric Storage Apparatus  62   
     3-4: Configuration of Interface Apparatus  63   
     3-5: Operation for Transmitting Electric Energy 
     4: Example of Hardware Configuration 
     5: Conclusion 
     1: First Embodiment (Issuing a Green Certificate) 
     Hereinafter, an explanation will be given regarding the first embodiment. 
     (1-1: Configuration of Home Energy Network  1 ) 
     At first, an explanation will be given regarding a configuration of a home energy network  1  according to the first embodiment with reference to  FIG. 1 .  FIG. 1  is an explanatory diagram for illustrating a configuration of a green energy generation system according to the first embodiment. Note that the word “household” will be used so that a concrete configuration can be grasped easily, and the word “household” indicates a size that an individual electric energy consumer can handle by himself. 
     As shown in  FIG. 1 , the home energy network  1  includes an energy consuming device  10 , a green energy generation apparatus  11 , an electric storage apparatus  12 , and an interface apparatus  13 . The home energy network  1  is connected to a community energy network  2  and an electric utility company  3  via energy lines. Further, in this specification, the green energy generation apparatus  11 , the electric storage apparatus  12 , and the interface apparatus  13  may be collectively called a green energy generation apparatus. 
     The energy consuming device  10  is a device that consumes electric energy. For example, the energy consuming device  10  may be a home electric appliance, such as a refrigerator, a washing machine, an air conditioner, an electric fan, an electric heating appliance, a computer, a video receiver, a video recording/reproducing device, a music player, an audio recorder, a lighting equipment, an electric stove, a microwave oven, a dishwasher, a ventilating fan, a drier, an electric toothbrush, or the like. 
     The green energy generation apparatus  11  implements a method for generating electricity using renewable energy or environmentally friendly resources. For example, the green energy generation apparatus  11  may implement a method for generating electricity using solar energy, wind energy, geothermal energy, water energy, atomic energy, biomass fuel, hydrogen fuel, or the like. This specification, however, will focus on a photovoltaic energy generation method for convenience. 
     The electric storage apparatus  12  implements a method for charging electric energy. The electric storage apparatus  12  implements a method for charging electric energy using, for example, a lead-acid battery, a lithium ion secondary battery (Li-Ion battery), a sodium-sulfur battery (NaS battery), an electric double-layer capacitor, a pumped hydroelectric storage system, or the like. This specification, however, will focus on the lithium ion secondary battery for convenience. 
     The interface apparatus  13  implements a method for allowing an electric energy consumer (hereinafter, a user) to input information, or for displaying information to the user. Note that the interface apparatus  13  may be incorporated into the green energy generation apparatus  11 , and the electric storage apparatus  12 . Moreover, the green energy generation apparatus  11  and the electric storage apparatus  12  may be combined as a unit. Further, the green energy generation apparatus  11 , the electric storage apparatus  12 , and the interface apparatus  13  may be combined as a unit. 
     An example shown in  FIG. 1  the energy consuming device  10  may operate upon a supply of electric energy directly from the green energy generation apparatus  11  or a supply of electric energy charged in the electric storage apparatus  12 . Generally, electricity generated using renewable energy tends to be unstable. It is preferable for the energy consuming device  10  to utilize electric energy that has been charged once in the electric storage apparatus  12 . For this reason, this specification will focus on an energy consuming device  10  that operates upon a supply of electricity charged in the electric storage apparatus  12 . 
     The electricity generated by the green energy generation apparatus  11  is to be stored in the electric storage apparatus  12 . The electric storage apparatus  12  can also store electricity supplied by the electric utility company  3 . The electricity stored in the electric storage apparatus  12  is to be supplied to the energy consuming device  10 , as described above. The electricity stored in the electric storage apparatus  12  is to be supplied to the community energy network  2  and the electric utility company  3 . For example, surplus electricity generated by the green energy generation apparatus  11  that has not been consumed by the energy consuming device  10  is to be sold to the electric utility company  3 . Such surplus electricity may be supplied to the community energy network  2  and may be sold to other users who form the community energy network  2 . 
     When the user has bought electricity from another user who forms the community energy network  2 , the electricity bought is to be stored in the electric storage apparatus  12 . The community energy network  2  is a kind of electric energy community that is formed by connecting a plurality of the home energy networks  1  to the energy network  20 . Note that the term “community” herein does not necessarily mean divisions, such as prefectures, municipalities, or the like, but a quantity of households being managed in the energy network  20 . Needless to say, the community energy network  2  may be divided by prefectures, municipalities, administrative areas managed by a distinct electric utility company  3 , or the like. Note that  FIG. 1  shows a detailed configuration of home energy network  1  outside of the community energy network  2  for the sake of simplicity; however, this home energy network  1  is supposed to be included in the community energy network  2 . 
     As described above, the electricity stored in the electric storage apparatus  12  will not only be consumed by the energy consuming device  10  that is included in the home energy network  1 , but will also be supplied to the electric utility company  3  or the community energy network  2 . Moreover, the electricity bought from another home energy network  1  in the community energy network  2 , or the electric utility company  3 , is to be stored in the electric storage apparatus  12 . Thus, in the system according to the present embodiment, electricity is to be received/transmitted both outside and inside of the home energy network  1 . User operations regarding such reception/transmission of electricity are performed via the interface apparatus  13 . For example, an operation to buy electricity from the electric utility company  3  or another home energy network  1  is performed using the interface apparatus  13 . 
     For this reason, the interface apparatus  13  is connected to another home energy network  1  or the electric utility company  3  via a communication network (not shown), and can exchange information with another home energy network  1  or the electric utility company  3 . The interface apparatus  13  can acquire information on the amount of stored electricity or on the amount of space, or the like, from the electric storage apparatus  12 , and can display the acquired information. The interface apparatus  13  can acquire information on electric-generation capacity from the green energy generation apparatus  11 . Further, the interface apparatus  13  can acquire a later-described certificate of electric-generation from the green energy generation apparatus  11 , and a later-described certificate of electric-storage from the electric storage apparatus  12 . 
     The certificate of electric-generation recited above is a digital certificate that authenticates electricity generated by green energy generation. This certificate of electric-generation is issued by the green energy generation apparatus  11 . The certificate of electric-storage is also a digital certificate that authenticates electricity generated by green energy generation, similar to the certificate of electric-generation. However, while the certificate of electric-generation is based on the electricity output from the green energy generation apparatus  11 , the certificate of electric-storage is based on the electricity output from the electric storage apparatus  12 . In other words, the certificate of electric-storage is based on the electricity generated by green energy generation modified by the electricity that has been lost through storage. 
     A digital certificate may include information, a digital signature based on the information, and/or a public key for verification of a digital signature. This public key is certified by a reliable certificate authority. In the example of  FIG. 1 , a certificate authority  4  corresponds to the certificate authority that certifies the public key. Further, the green energy generation apparatus  11  and the electric storage apparatus  12  are assumed to hold a private key and a public key generated in advance for issuance of the digital certificate. The public key is assumed to be certified by the certificate authority  4 . 
     As described above, the green energy generation apparatus  11  holds the private key paired with the public key that is certified by the certificate authority  4 . Utilizing this private key, the green energy generation apparatus  11  issues a certificate of electric-generation that authenticates information on electric-generation capacity when supplying the generated electricity to the electric storage apparatus  12 . Specifically, the green energy generation apparatus  11  generates a digital signature based on the information on the electric-generation capacity by utilizing the private key, and generates a certificate of electric-generation with the addition of a digital signature to the information on the electric-generation capacity. Note that information on an area where the electricity has been generated (information that identifies the community energy network  2 ) may be added to the certificate of electric-generation. 
     The certificate of electric-generation issued by the green energy generation apparatus  11  is to be input into the electric storage apparatus  12 . As described above, the electric storage apparatus  12  holds a private key paired with a public key that has been certified by the certificate authority  4 , and the electric storage apparatus  12  uses this private key to issue a certificate of electric-storage. At first, the electric storage apparatus  12  acquires a public key certificate corresponding to the green energy generation apparatus  11  from the certificate authority  4 , and verifies a digital signature contained in the certificate of electric-generation by using a public key contained in the public key certificate. 
     If successful in verifying the digital signature, the electric storage apparatus  12  multiplies electric-generation capacity contained in the certificate of electric-generation by storage efficiency to calculate electric-generation capacity modified based on electricity which has been lost through storage (hereinafter, referred to as modified electric-generation capacity). Subsequently, the electric storage apparatus  12  issues a certificate of electric-storage that authenticates information on the modified electric-generation capacity. Specifically, the electric storage apparatus  12  generates a digital signature based on the information on the modified electric-generation capacity by utilizing the private key, and generates a certificate of electric-storage with the addition of a digital signature to the information on the modified electric-generation capacity. Note that information on an area where the electricity has been generated (information that identifies the community energy network  2 ) may be added to the certificate of electric-storage. 
     The certificate of electric-storage issued as above is to be input into the interface apparatus  13 . The interface apparatus  13  provides the certificate of electric-storage that has been input by the electric storage apparatus  12  to another home energy network  1  or electric utility company  3  when supplying electricity from the electric storage apparatus  12  to the other home energy network  1  or the electric utility company  3 . By providing the certificate of electric-storage in this way, the interface apparatus  13  verifies that the electricity provided from the electric storage apparatus  12  is green energy. 
     Thus, the green energy generation system according to the present embodiment utilizes the certificate of electric-generation to provide a structure for verifying that the electricity generated by the green energy generation apparatus  11  is green energy. By using such a structure, when there is a gap between a green energy price and a standard energy price, for example, providing a certificate of electric-storage makes it possible to sell the electricity from the electric storage apparatus  12  at the green energy price. 
     Moreover, even in a case where the electricity generated by the green energy generation apparatus  11  has been consumed within the home energy network  1 , a certificate of electric-generation remains if nothing else. The remaining certificate of electric-generation makes it possible to sell any arbitrary electricity bought from the electric utility company  3  at the green energy price. Thus, a benefit may be gained by generating green energy, whether or not the generated green energy is sold. Providing such a structure makes the monetary value of the green energy generation apparent, and it is expected that this would encourage the users to invest in equipment for green energy generation. Moreover, if the above structure is realized, the certificate of electric-generation itself gains monetary value and the certificate of electric-generation can be securitized. 
     Thus, to corporealize the value of green energy is extremely meaningful for the purpose of boosting the user&#39;s awareness of the green energy generation. If the value of green energy generation is widely-recognized, it accelerates the investment in equipment for green energy generation, and consequently, it will decrease the emission of greenhouse gases. Building a structure to combine the value of green energy and monetary value is expected to lead to the improvement of the world environment. The overall picture of the green energy generation system has been described hereto. A detailed configuration of the green energy generation system will be described later. 
     The configuration of the home energy network  1  according to the present embodiment has been explained as above. 
     (1-2: Configuration of Green Energy Generation Apparatus  11 ) 
     With reference to  FIG. 2  and  FIG. 3 , an explanation will be given regarding configurations of the green energy generation apparatus  11  according to the present embodiment.  FIG. 2  is an explanatory diagram for illustrating a functional configuration of the green energy generation apparatus  11  according to the embodiment of the present invention.  FIG. 3  is an explanatory diagram for illustrating a functional configuration of a certificate of electric-generation issuance unit  114  according to the embodiment of the present invention. 
     (Overall Configuration) 
     As shown in  FIG. 2 , the green energy generation apparatus  11  includes a photovoltaic panel  111 , an electric-generation capacity measurement unit  112 , an electric energy transmission unit  113 , a certificate of electric-generation issuance unit  114 , and a communication unit  115 . Note that in the following description, the electric-generation capacity measurement unit  112  and the certificate of electric-generation issuance unit  114  may be collectively called a certificate module. 
     The electric-generation capacity measurement unit  112 , certificate of electric-generation issuance unit  114 , and communication unit  115  may be implemented as software modules. The green energy generation apparatus  11  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the electric-generation capacity measurement unit  112 , certificate of electric-generation issuance unit  114 , and/or communication unit  115  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     The photovoltaic panel  111  implements a method for generating energy by converting the energy of solar light into electricity. The photovoltaic panel  111  utilizes, for example, silicon solar cells or dye-sensitized solar cells, or the like. The electricity generated by the photovoltaic panel  111  is to be input into the electric-generation capacity measurement unit  112 . The electric-generation capacity measurement unit  112  measures the electric-generation capacity of the photovoltaic panel  111 . Information on the electric-generation capacity measured by the electric-generation capacity measurement unit  112  is to be input into the certificate of electric-generation issuance unit  114 . 
     The electricity generated by the photovoltaic panel  111  is to be input into the electric energy transmission unit  113  via the electric-generation capacity measurement unit  112 . The electric energy transmission unit  113 , to which the electricity generated by the photovoltaic panel  111  has been input, transmits the input electricity to the energy consuming device  10  or the electric storage apparatus  12 . Note that in this description herein, the electricity input to the electric energy transmission unit  113  is to be transmitted to the electric storage apparatus  12 . 
     The certificate of electric-generation issuance unit  114 , to which the information on the electric-generation capacity has been input from the electric-generation capacity measurement unit  112 , issues a certificate of electric-generation comprising the input information on the electric-generation capacity. Note that the detailed functional configuration of the certificate of electric-generation issuance unit  114  will be described later. The certificate of electric-generation that has been issued by the certificate of electric-generation issuance unit  114  is to be input into the communication unit  115 . The communication unit  115 , to which the certificate of electric-generation has been input, transmits the input certificate of electric-generation to the electric storage apparatus  12 . 
     (Details of the Certificate of Electric-Generation Issuance Unit  114 ) 
     An explanation will be given regarding the detailed functional configuration of the certificate of electric-generation issuance unit  114 . As shown in  FIG. 3 , the certificate of electric-generation issuance unit  114  includes a signature generation unit  1141 , a storage unit  1142 , and a certificate generation unit  1143 . Note that the storage unit  1142  stores a private key and a corresponding public key, which have been generated in advance. Further, the public key stored in the storage unit  1142  is assumed to have been certified by the certificate authority  4 . 
     The signature generation unit  1141 , storage unit  1142 , and certificate generation unit  1143  may be implemented as software modules. The green energy generation apparatus  11  or certificate of electric-generation issuance unit  114  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the signature generation unit  1141 , storage unit  1142 , and/or certificate generation unit  1143  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     In the event when the information on the electric-generation capacity is input from the electric-generation capacity measurement unit  112  to the certificate of electric-generation issuance unit  114 , the input information on the electric-generation capacity is to be input into the signature generation unit  1141 . The signature generation unit  1141 , to which the information on the electric-generation capacity has been input, acquires a private key from the storage unit  1142 , and generates a digital signature based on the information on the electric-generation capacity using the private key that has been acquired. The digital signature generated by the signature generation unit  1141  is to be input into the certificate generation unit  1143  along with the information on the electric-generation capacity. The certificate generation unit  1143 , to which the digital signature and the information on the electric-generation capacity have been input, generates a certificate of electric-generation containing the digital signature and the information on the electric-generation capacity which have been input. Subsequently, the certificate of electric-generation generated by the certificate generation unit  1143  is to be input into the communication unit  115 . 
     (Content of a Certificate of Electric-Generation) 
     In this example, a certificate of electric-generation contains information on electric-generation capacity and a digital signature. By using this certificate of electric-generation, it is possible to authenticate the electric-generation capacity of the electricity that has been generated from green electric energy generation. However, there are some cases where it is desirable to add information regarding type of green energy generation and area of energy generation, or the like. 
     For example, when the energy price varies depending on the amount of emission of greenhouse gas generated at the time of energy generation, it is required to know the type of the green energy generation in order to determine the energy price for trading. In such a case, it is preferable to add information that indicates the type of the green energy generation (hereinafter, referred to as the electric energy generation method data) in the certificate of electric-generation. In the examples of  FIG. 2  and  FIG. 3 , the electric energy generation method data that indicates photovoltaic generation is added to the certificate of electric-generation. In the case of a generation method utilizing renewable sources, the amount of emission of greenhouse gas is nearly zero; however, in the case of environmentally friendly energy generation, the emission amount of greenhouse gases varies depending on the fuel used for electric energy generation. In this case, types of fuels are to be indicated in the electric energy generation method data. It may be possible to develop a reference index indicating a degree of environmental load at the time of energy generation, and to add information indicating that reference index as the electric energy generation method data. 
     If (as a regional policy) it is expected to have a type of energy consumption that is locally produced and locally consumed, it is preferable to add information on area of generation (hereinafter, referred to as the area information) to a certificate of electric-generation. If the area of generation is away from the area of consumption geographically, it causes energy transmission loss when the electric energy is transmitted through energy transmission lines. Moreover, performing repeatedly DC/AC conversion and frequency conversion causes energy loss. For these reasons, in many cases, the type of energy consumption that is locally produced and consumed locally is desired. In the event of promoting such energy consumption type, generally a policy is applied so that the selling price of electricity locally produced and locally consumed is set high, and buying price of electricity locally produced and locally consumed is set low. Then, the certificate of electric-generation is expected to be added with the area information in order to authenticate that the electricity is locally produced and locally consumed. Information on a desired area of consumption may be added as the area information. 
     By adding such information to a certificate of electric-generation, the value of the certificate of electric-generation can be determined more precisely, and this contributes to the promotion of the green electric energy generation equipment having the best impact on the environment. It also contributes to the development of the regional policy and regional services, or to the formation of a trading market which is region-oriented. Note that it is desirable that various kinds of information added to the certificate of electric-generation be reliably authenticated with a digital signature, similar to the information on electric-generation capacity. In this case, the generation of the digital signature is performed by the signature generation unit  1141 . Moreover, the generation of a certificate containing various kinds of information and the corresponding digital signature is performed by the certificate generation unit  1143 . 
     The configuration of the green energy generation apparatus  11  according to the present embodiment has been explained above. 
     (1-3: Configuration of Electric Storage Apparatus  12 ) 
     Next, with reference to  FIG. 4  and  FIG. 5 , the configuration of the electric storage apparatus  12  according to the present embodiment will be explained.  FIG. 4  is an explanatory diagram for illustrating a functional configuration of the electric storage apparatus  12  according to the present embodiment.  FIG. 5  is an explanatory diagram for illustrating a functional configuration of the certificate of electric-storage issuance unit  125  according to the present embodiment. 
     (Overall Configuration) 
     As shown in  FIG. 4 , the electric storage apparatus  12  includes an electric energy receiving unit  121 , a charge/discharge control unit  122 , a battery  123 , an electric energy transmission unit  124 , a certificate of electric-storage issuance unit  125 , and a communication unit  126 . 
     The certificate of electric-storage issuance unit  125  and communication unit  126  may be implemented as software modules. The electric storage apparatus  12  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the certificate of electric-storage issuance unit  125  and/or communication unit  126  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     (At Receiving) 
     At first, the electricity supplied by the green energy generation apparatus  11  is to be received by the electric energy receiving unit  121 . The electricity received by the electric energy receiving unit  121  is to be input into the charge/discharge control unit  122 . The charge/discharge control unit  122 , to which the electricity has been input by the electric energy receiving unit  121 , inputs the input electricity to the battery  123  so as to charge the battery  123 . At this time, the certificate of electric-storage issuance unit  125  receives a certificate of electric-generation from the green energy generation apparatus  11  via the communication unit  126 . The certificate of electric-storage issuance unit  125 , which has received the certificate of electric-generation, issues a certificate of electric-storage based on the received certificate of electric-generation and the storage efficiency of the battery  123 . Note that the detailed description of the functional configuration of the certificate of electric-storage issuance unit  125  will be described later. 
     (At Transmitting) 
     If an instruction for discharging is received from the interface apparatus  13  via the communication unit  126 , the instruction is to be input into the charge/discharge control unit  122  via the communication unit  126 . The charge/discharge control unit  122 , to which the instruction for discharging has been input, discharges from the battery  123  the amount of electricity based on the instruction. The electricity that has been discharged from the battery  123  by the charge/discharge control unit  122  is to be input into the electric energy transmission unit  124 , and is to be transmitted to the energy consuming device  10 , the community energy network  2 , or the electric utility company  3  from the electric energy transmission unit  124 . In this example, however, it is assumed to be transmitted to the electric utility company  3 . At this time, the charge/discharge control unit  122  inputs information on the amount of discharged electricity into the certificate of electric-storage issuance unit  125 . 
     The certificate of electric-storage issuance unit  125 , to which the information on the amount of discharged electricity has been input, based on the input information on the amount of discharged electricity, generates a certificate of electric-storage that authenticates the amount of discharged electricity generated from green energy generation. If it is possible to use a certificate of electric-storage issued at the time of receiving the electricity from the green energy generation apparatus  11 , the certificate of electric-storage will be used. In this example, however, it is assumed that the certificate of electric-storage issuance unit  125  issues a certificate of electric-storage corresponding to the amount of the discharged electricity. The certificate of electric-storage issuance unit  125 , which has issued the certificate of electric-storage corresponding to the amount of discharged electricity, transmits the certificate of electric-storage to the interface apparatus  13  via the communication unit  126 . 
     (Detail Description of the Certificate of Electric-Storage Issuance Unit  125 ) 
     The detailed functional configuration of the certificate of electric-storage issuance unit  125  will be explained. As shown in  FIG. 5 , the certificate of electric-storage issuance unit  125  includes a signature verification unit  1251 , an electric-generation capacity modification unit  1252 , a signature generation unit  1253 , a storage unit  1254 , and a certificate generation unit  1255 . Note that the storage unit  1254  stores a private key and a corresponding public key which have been generated in advance. Further, the public key stored in the storage unit  1254  is assumed to have been certified by the certificate authority  4 . 
     The signature verification unit  1251 , electric-generation capacity modification unit  1252 , signature generation unit  1253 , storage unit  1254 , and certificate generation unit  1255  may be implemented as software modules. The electric storage apparatus  12  or the certificate of electric-storage issuance unit  125  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the signature verification unit  1251 , electric-generation capacity modification unit  1252 , signature generation unit  1253 , storage unit  1254 , and/or certificate generation unit  1255  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     (At Receiving) 
     At first, the certificate of electric-generation received by the certificate of electric-storage issuance unit  125  via the communication unit  126  is to be input into the signature verification unit  1251 . The signature verification unit  1251 , to which the certificate of electric-generation has been input, acquires a public key certificate corresponding to the green energy generation apparatus  11  from the certificate authority  4  via the communication unit  126 . Then, the signature verification unit  1251  uses the public key contained in the public key certificate to verify the validity of the digital signature contained in the certificate of electric-generation. If successful in the verification of the digital signature, the signature verification unit  1251  inputs the information on the electric-generation capacity contained in the certificate of electric-generation into the electric-generation capacity modification unit  1252 . 
     If area information or the like is added to the certificate of electric-generation, the signature verification unit  1251  verifies the digital signature in the same way, and if successful, the signature verification unit  1251  inputs the area information or the like into the certificate generation unit  1255 . In this example, however, it is assumed that only information on the electric-generation capacity is contained in the certificate of electric-generation. 
     The electric-generation capacity modification unit  1252 , to which the information on the electric-generation capacity has been input by the signature verification unit  1251 , acquires information that indicates the charge efficiency of the battery  123  from the charge/discharge control unit  122 . Then, the electric-generation capacity modification unit  1252  multiplies the electric-generation capacity by the charge efficiency of the battery  123  to calculate the modified electric-generation capacity, adjusted based on the loss of electricity in the battery  123 . Note that if the electric-generation capacity indicated by the certificate of electric-generation is used as it is, the value of the electricity lost in the battery  123  (the electricity lost without being consumed) remains unaccounted. For this reason, the electric-generation capacity is to be modified by the electric-generation capacity modification unit  1252 . 
     The information on the modified electric-generation capacity calculated by the electric-generation capacity modification unit  1252  is to be input into the signature generation unit  1253 . The signature generation unit  1253 , to which the information on the modified electric-generation capacity has been input, acquires a private key from the storage unit  1254 , and generates a digital signature based on the information on the modified electric-generation capacity using the private key that has been acquired. The digital signature generated by the signature generation unit  1253  is to be input into the certificate generation unit  1255  along with the information on the modified electric-generation capacity. The certificate generation unit  1255 , to which the digital signature and the information on the modified electric-generation capacity have been input, generates a certificate of electric-storage containing the digital signature and the information on the modified electric-generation capacity which have been input. 
     Note that if area information or the like is contained in the certificate of electric-generation, the signature generation unit  1253  generates a digital signature for the area information or the like, and the certificate generation unit  1255  generates a certificate of electric-storage containing area information or the like. Note that the certificate generation unit  1255  may include the area information or the like that is contained in the certificate of electric-generation in the certificate of electric-storage as it is. 
     (At Transmitting) 
     If the electric storage apparatus  12  receives a discharge instruction from the interface apparatus  13  via the communication unit  126 , information on the amount of discharged electricity is input to the certificate of electric-storage issuance unit  125 . The information on the amount of discharged electricity that has been input into the certificate of electric-storage issuance unit  125  is to be input into the certificate generation unit  1255 . The certificate generation unit  1255 , to which the information on the amount of discharged electricity has been input, generates a certificate of electric-storage corresponding to the amount of discharged electricity to transmit to the discharging destination (the electric utility company  3  in this example) via the communication unit  126 . Note that there are two methods for generating the certificate of electric-generation corresponding to the amount of discharged electricity as follows. 
     The first method generates a plurality of certificates of electric-storage corresponding to a prescribed amount of electricity (modified electric-generation capacity) per unit at the time of receiving the electricity. In other words, when receiving one certificate of electric-generation corresponding to a certain amount of electric-generation capacity, this method does not generate only one certificate of electric-storage corresponding to the modified electric-generation capacity. Rather, the necessary amount of certificates of electric-storage are generated in accordance with the prescribed amount of electricity per unit. Therefore, if this method is applied, at the time of receiving the electricity, as many as (the modified electric-generation capacity/the amount of electricity per unit) certificates of electric-storage will be generated. If certificates of electric-storage are generated piece by piece for each unit of electricity and are stored in the storage unit  1254 , the number of certificates of electric-storage provided at the time of discharging electricity corresponds to the amount of discharged electricity output to the discharging destination. 
     The second method generates one certificate of electric-storage corresponding to the amount of discharged electricity at the time of discharging electricity. Upon receiving one certificate of electric-generation corresponding to a certain amount of electric-generation capacity, this method generates one certificate of electric-storage corresponding to the modified electric-generation capacity, and stores the certificate of electric-storage in the storage unit  1254 . At the time of discharging electricity, the certificate of electric-storage stored in the storage unit  1254  is read out, the modified electric-generation capacity is divided into the first modified electric-generation capacity, which equals the amount of discharged electricity, and the second modified electric-generation capacity, which is calculated by subtracting the first modified electric-generation capacity from the original modified electric-generation capacity, and certificates of electric-storage corresponding to each of the first and the second modified electric-generation capacities are generated. Subsequently, the certificate of electric-storage for the first modified electric-generation capacity will be provided to the discharging destination, while the certificate of electric-storage for the second modified electric-generation capacity will be stored in the storage unit  1254 . 
     Using either method, a certificate of electric-storage corresponding to the amount of discharged electricity will be acquired and the certificate of electric-storage will be provided to the discharging destination via the communication unit  126 . It is not necessary to recreate the certificates of electric-storage when using the first method described above; however, when using the second method described above, it is necessary to recreate the certificates of electric-storage (creating the first and the second certificates of electric-storage). In this case, a process to recreate the certificate of electric-storage will be performed by the signature generation unit  1253  and the certificate generation unit  1255 , similar to the process of creating the certificate of electric-storage performed at the time of receiving electricity. Note that the two methods described herein are examples and other methods may be applied to create certificates of electric-storage corresponding to the amount of discharged electricity. In this specification, it is assumed that certificates of electric-storage corresponding to the amount of discharged electricity are generated by the second method. 
     The configuration of the electric storage apparatus  12  according to the present embodiment has been explained as above. 
     (1-4: Configuration of Interface Apparatus  13 ) 
     With reference to  FIG. 6 , the configuration of the interface apparatus  13  according to the present embodiment will be explained.  FIG. 6  is an explanatory diagram for illustrating a functional configuration of an interface apparatus according to the embodiment of the present invention. 
     As shown in  FIG. 6 , the interface apparatus  13  includes a communication unit  131 , a central processing unit  132 , a storage unit  133 , a display unit  134  and an input unit  135 . 
     When the electricity is discharged from the electric storage apparatus  12 , a user inputs information on the amount of discharged electricity or the like using the input unit  135 . The information that has been input using the input unit  135  is to be input into the electric storage apparatus  12  via the central processing unit  132  and the communication unit  131 . When the process of discharging electricity is performed by the electric storage apparatus  12 , the communication unit  131  receives a certificate of electric-storage. The certificate of electric-storage received by the communication unit  131  is to be input into the central processing unit  132 . The central processing unit  132 , to which the certificate of electric-storage has been input, transmits the input certificate of electric-storage to a discharging destination (the electric utility company  3  in this example) via the communication unit  131 . 
     Note that the interface apparatus  13  allows the user to input or display information when the user buys or sells the electricity. The process for realizing such functions is performed by the central processing unit  132 . For example, the central processing unit  132  acquires information on the buying price of electricity and the selling price of electricity from the electric utility company  3  or the like via the communication unit  131 , and displays the information on the display unit  134 . Moreover, the central processing unit  132  inputs the information on the amount of electricity to buy or the amount of electricity to sell, which has been input using the input unit  135 , into the display unit  134 , or transmits the information to the electric utility company  3  or the like via the communication unit  131 . 
     Further, when the user inputs an instruction to sell electricity using the input unit  135 , the central processing unit  132  transmits an instruction to discharge electricity and information on the amount of electricity to the electric storage apparatus  12  via the communication unit  131 . Note that the storage unit  133  is used to store information received from the electric storage apparatus  12  or the electric utility company  3 , or information that a user has input, or used to store a public key or the like corresponding to the green energy generation apparatus  11  or the electric storage apparatus  12  as needed. The storage unit  133  is used to store a program that defines operations of the central processing unit  132 . 
     The configuration of the interface apparatus  13  according to the present embodiment has been explained as above. 
     2: Second Embodiment (Structure of Energy Trading Market for Individual Consumer) 
     Hereinafter, a second embodiment will be explained. Note that structural elements with functions and structures that are substantially the same as those of the structural elements according to the first embodiment described above are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     (2-1: Configuration of Energy Trade System) 
     First, with reference to  FIG. 7 , the configuration of the energy trade system according to the present embodiment will be explained.  FIG. 7  is an explanatory diagram for illustrating a functional configuration of an energy trade server according to the embodiment. Note that the word “household” will be used, as in the description of the first embodiment, so that a concrete configuration can be grasped easily, and w the word “household” herein indicates a size that an individual electric utility consumer can handle by himself. 
     As shown in  FIG. 7 , the energy trade system according to the present embodiment includes a home energy network  1 , a plurality of community energy networks  2  (the first community energy network to the N-th community energy network), a certificate authority  4  and an energy trade server  5 . Note that an electric utility company  3  may be included, similar to  FIG. 1 . Moreover, the home energy network  1  includes the energy consuming device  10 , the green energy generation apparatus  11 , the electric storage apparatus  12  and the interface apparatus  13 . The home energy network  1  is assumed to be connected to a plurality of the community energy networks  2  (the first community energy network to the N-th community energy network) via energy lines. 
     The major difference between  FIG. 1  and  FIG. 7  is the existence of the energy trade server  5 . As described in the above explanation of the first embodiment, there is a price gap between ordinary electric energy, which has been produced using fossil fuels or the like, and green energy, and if a structure is developed in which the electricity can be traded at the price of green energy by adding a certificate of electric-generation or a certificate of electric-storage to a certain electric energy, the certificate of electric-generation and the certificate of electric-storage will have monetary value. Further, it will be possible to develop a market where the certificate of electric-generation or the certificate of electric-storage itself can be traded independently after being securitized. 
     The energy trade server  5  described above is a server apparatus for managing energy trade, such as the one described above, and for managing trade of the certificate of electric-generation or the certificate of electric-storage, or the like. The present embodiment relates to a method for managing such energy trade, and trade of the certificate of electric-generation or the certificate of electric-storage, or the like. Hereinafter, functions of the electric storage apparatus  12  added to the configuration of the first embodiment described above, functions of the interface apparatus  13 , and functions of the energy trade server  5 , which realize management of the e trade according to the present embodiment, will be explained one by one. 
     Although the certificate of electric-generation and the certificate of electric-storage differ as to whether electricity loss occurring at the time of storage has been taken into consideration, they are substantially identical in that they are both digital certificates that authenticate green energy. When the loss at the time of storage is small enough to be negligible, the certificate of electric-generation and the certificate of electric-storage become the same. For this reason, in the following description, it is assumed that the certificate of electric-storage will be used for energy trade. 
     The configuration of the energy trade according to the present embodiment has been explained above. 
     (2-2: Configuration of Electric Storage Apparatus  12 ) 
     With reference to  FIG. 8  through  FIG. 10 , the configuration of the electric storage apparatus  12  according to the present embodiment will be explained.  FIG. 8  is an explanatory diagram for illustrating a functional configuration of the electric storage apparatus  12  according to the present embodiment.  FIG. 9  is an explanatory diagram for illustrating a functional configuration of a certificate of remaining issuance unit  127  according to the present embodiment.  FIG. 10  is an explanatory diagram for illustrating a functional configuration of a certificate of space issuance unit  128  according to the present embodiment. 
     As shown in  FIG. 8 , the electric storage apparatus  12  includes the electric energy receiving unit  121 , the charge/discharge control unit  122 , the battery  123 , the electric energy transmission unit  124 , the certificate of electric-storage issuance unit  125 , the communication unit  126 , the certificate of remaining issuance unit  127  and the certificate of space issuance unit  128 . Note that functions of the electric energy receiving unit  121 , the charge/discharge control unit  122 , the battery  123 , the electric energy transmission unit  124 , the certificate of electric-storage issuance unit  125 , and the communication unit  126  are the same as the electric storage apparatus  12  according to the first embodiment described above; therefore, detailed explanation regarding these structural elements will be omitted and an explanation will be given regarding the certificate of remaining issuance unit  127  and the certificate of space issuance unit  128  only. 
     The certificate of electric-storage issuance unit  125 , communication unit  126 , certificate of remaining issuance unit  127 , and certificate of space issuance unit  128  may be implemented as software modules. The electric storage apparatus  12  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the certificate of electric-storage issuance unit  125 , communication unit  126 , certificate of remaining issuance unit  127 , and/or certificate of space issuance unit  128  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     (Introduction) 
     In addition to energy, there exist various types of commodities being traded in the world. For example, various financial commodities are circulating in the market, such as interest, currency, loan receivable, securities, or the like. Moreover, derivatives, such as futures, swaps, options, or the like are also widely utilized. For example, futures are contracts to trade cash commodities on a future date at a currently specified price. The cash commodities include various types of commodities, such as oil, energy, emission credit, wheat, soybeans, corn, or the like. 
     Originally, futures used to be executed on a due date being associated with delivery and receipt of the cash commodities. Today, however, transactions involving futures generally do not require actual commodities to be delivered or received, and the offsetting transaction is executed by the due date for offsetting. For this reason, transaction prices of cash commodities often move up and down rapidly due to speculative transactions. 
     However, the energy trading market assumed in the present embodiment is a market individual electric utility consumers join based on the assumption that cash commodities are to be delivered and received. Therefore, it is strongly desired to eliminate risks, such as a gyrating market price caused by those speculative transactions, or sagging energy supply on the delivery date. To eliminate such risks, it is necessary to exclude players who repeatedly sell and buy in a period of time shorter than the necessary time for the delivery of commodities, to exclude players who do not own any cash commodities to be delivered, or to exclude players who have difficulty receiving cash commodities. 
     Although futures have been described as an example here, the present embodiment is directed to a real-time energy trade. Therefore, it is preferable that the energy trade server  5  manages information in real time, the information including, for example, whether a player who has placed a sell order currently has enough remaining battery level to deliver, or whether a player who has placed a buy order has enough battery space to receive. However, as the number of players increases, it will be difficult for the energy trade server to determine each player&#39;s remaining battery level and battery space in real time. The present embodiment resolves such issues. 
     (Functions of the Certificate of Remaining Issuance Unit  127 ) 
     The energy trade system according to the present embodiment utilizes a certificate of remaining that authenticates remaining battery level in order to verify the remaining battery level of a player who has placed a sell order. This certificate of remaining is issued by the certificate of remaining issuance unit  127  of the electric storage apparatus  12 . 
     As shown in  FIG. 9 , the certificate of remaining issuance unit  127  includes a remaining acquisition unit  1271 , a signature generation unit  1272 , a storage unit  1273  and a certificate generation unit  1274 . Note that the storage unit  1273  stores a private key and a corresponding public key which have been generated in advance. Further, the public key stored in the storage unit  1273  is assumed to have been certified by the certificate authority  4 . 
     The remaining acquisition unit  1271 , signature generation unit  1272 , storage unit  1273 , and certificate generation unit  1274  may be implemented as software modules. The electric storage apparatus  12  or the certificate of remaining issuance unit  127  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the remaining acquisition unit  1271 , signature generation unit  1272 , storage unit  1273 , and/or certificate generation unit  1274  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     If a user places a sell order using the interface apparatus  13 , a notification indicating the sell order is to be input into the charge/discharge control unit  122  via the communication unit  126 . The charge/discharge control unit  122  receiving this notification detects remaining battery level of the battery  123 , and inputs information on the detected remaining battery level into the certificate of remaining issuance unit  127 . The information on the remaining battery level, which has been input into the certificate of remaining issuance unit  127 , will be acquired by the remaining acquisition unit  1271 . And the information on the remaining battery level acquired by the remaining acquisition unit  1271  is to be input into the signature generation unit  1272 . 
     The signature generation unit  1272 , to which the information on the remaining battery level has been input, acquires a private key from the storage unit  1273 , and generates a digital signature based on the information on the remaining battery level using the private key that has been acquired. The digital signature generated by the signature generation unit  1272  is to be input into the certificate generation unit  1274  along with the information on the remaining battery level. The certificate generation unit  1274 , to which the digital signature and the information on the remaining battery level have been input, generates a certificate of remaining containing the digital signature and the information on the remaining battery level which have been input. Subsequently, the certificate of remaining generated by the certificate generation unit  1274  is to be input into the communication unit  126 . This certificate of remaining is to be transmitted to the interface apparatus  13  via the communication unit  126 , and to the energy trade server  5  via the interface apparatus  13 . 
     (Functions of the Certificate of Space Issuance Unit  128 ) 
     The energy trade system according to the present embodiment utilizes a certificate of space that authenticates battery space in order to verify the battery space of a player who has placed a buy order. This certificate of space is issued by the certificate of space issuance unit  128  of the electric storage apparatus  12 . 
     As shown in  FIG. 10 , the certificate of space issuance unit  128  includes a space acquisition unit  1281 , a signature generation unit  1282 , a storage unit  1283  and a certificate generation unit  1284 . Note that the storage unit  1283  stores a private key and a corresponding public key which have been generated in advance. Further, the public key stored in the storage unit  1283  is assumed to have been certified by the certificate authority  4 . 
     The space acquisition unit  1281 , signature generation unit  1282 , storage unit  1283 , and certificate generation unit  1284  may be implemented as software modules. The electric storage apparatus  12  or the certificate of space issuance unit  128  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the space acquisition unit  1281 , signature generation unit  1282 , storage unit  1283 , and/or certificate generation unit  1284  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     If a user places a buy order using the interface apparatus  13 , a notification indicating the buy order is to be input into the charge/discharge control unit  122  via the communication unit  126 . The charge/discharge control unit  122  receiving this notification detects battery space of the battery  123 , and inputs information on the detected battery space into the certificate of space issuance unit  128 . The information on the battery space, which has been input into the certificate of space issuance unit  128 , will be acquired by the space acquisition unit  1281 . And the information on the battery space acquired by the space acquisition unit  1281  is to be input into the signature generation unit  1282 . 
     The signature generation unit  1282 , to which the information on the battery space has been input, acquires a private key from the storage unit  1283 , and generates a digital signature based on the information on the battery space using the private key that has been acquired. The digital signature generated by the signature generation unit  1282  is to be input into the certificate generation unit  1284  along with the information on the battery space. The certificate generation unit  1284 , to which the digital signature and the information on the battery space have been input, generates a certificate of space containing the digital signature and the information on the battery space which have been input. Subsequently, the certificate of space generated by the certificate generation unit  1284  is to be input into the communication unit  126 . This certificate of space is to be transmitted to the interface apparatus  13  via the communication unit  126 , and to the energy trade server  5  via the interface apparatus  13 . 
     The configuration of the electric storage apparatus  12  has been explained. As described above, a certificate of remaining is to be transmitted to the energy trade server  5  when placing a sell order, while a certificate of space is to be transmitted to the energy trade server  5  when placing a buy order. The energy trade server  5  can limit the processing of buy orders or sell orders based on certificates of remaining and certificates of space which are being transmitted on a real-time basis. As a result, it may be possible to avoid the risk of cash commodities being unable to be delivered or received. 
     (2-3: Configuration of Interface Apparatus  13 ) 
     With reference to  FIG. 11 , and  FIG. 12 , a configuration of the interface apparatus  13  according to the present embodiment will be explained.  FIG. 11  is an explanatory diagram for illustrating a functional configuration of the interface apparatus  13  according to the present embodiment.  FIG. 12  is an explanatory diagram for illustrating a functional configuration of a certificate of consumption issuance unit  136  according to the present embodiment. 
     As shown in  FIG. 11 , the interface apparatus  13  includes the communication unit  131 , the central processing unit  132 , the storage unit  133 , the display unit  134  and the input unit  135 . Note that the communication unit  131 , the storage unit  133 , the display unit  134  and the input unit  135  are substantially the same as the interface apparatus  13  according to the first embodiment described; therefore, a detailed explanation regarding these structural elements will be omitted and an explanation will be given regarding the functions of the central processing unit  132  (the certificate of consumption issuance unit  136 ) only. 
     As described above, restricting processing of buy orders or sell orders based on a certificate of remaining or a certificate of space makes it possible to avoid the risk of cash commodities being unable to be delivered or received. However, the remaining battery level of the battery  123  is getting decreased every second. For that reason, it is preferable to allow a user to place a sell order up to an amount of electricity calculated by subtracting a certain margin from the remaining battery level that the certificate of remaining indicates. On the other hand, it is acceptable to allow a user to place a buy order up to the amount of electricity equal to the battery space that the certificate of space indicates. 
     Moreover, if a certain amount of electricity consumption is expected by the time of the actual delivery of the commodities, it is preferable to allow a user to place a sell order up to the upper limit of electricity based upon consideration of the certain amount of electricity consumption expected. The certificate of consumption issuance unit  136 , which is one of the functions of the central processing unit  132 , includes a function to issue a certificate of consumption that authenticates the amount of expected electricity consumption. 
     As shown in  FIG. 12 , the certificate of consumption issuance unit  136  includes a consumption setting unit  1361 , a signature generation unit  1362 , a storage unit  1363  and a certificate generation unit  1364 . Note that the storage unit  1363  stores a private key and a corresponding public key which have been generated in advance. Further, the public key stored in the storage unit  1363  is assumed to have been certified by the certificate authority  4 . 
     The consumption setting unit  1361  monitors the amount of electricity consumption of an energy consuming device  10 . The consumption setting unit  1361  detects an energy consuming device  10  which constantly consumes electricity, and calculates the amount of electricity consumption per unit of time consumed by the detected energy consuming device  10 . Note that if the user specifies the energy consuming device  10  by using the input unit  135 , the amount of electricity consumption per unit of time consumed by the specified energy consuming device  10  will be calculated. Then, when the user places a sell order using the input unit  135 , the consumption setting unit  1361  calculates the electricity to be consumed in the prescribed period of time before the actual delivery based on the amount of electricity consumption per unit of time that has been calculated in advance, and sets the result as the electricity consumption. 
     The information on the electric consumption that has been set by the consumption setting unit  1361  is to be input into the signature generation unit  1362 . The signature generation unit  1362 , to which the information on the electric consumption has been input, acquires a private key from the storage unit  1363 , and generates a digital signature based on the information on the electric consumption using the private key that has been acquired. The digital signature generated by the signature generation unit  1362  is to be input into the certificate generation unit  1364  along with the information on the electric consumption. The certificate generation unit  1364 , to which the digital signature and the information on the electric consumption have been input, generates a certificate of electric consumption containing the digital signature and the information on the electric consumption which have been input. The certificate of electric consumption generated by the certificate generation unit  1364  is to be input into the communication unit  131 , and is transmitted to the energy trade server  5  via the communication unit  131 . 
     The configuration of the interface apparatus  13  according to the present embodiment has been explained above. As described above, using the certificate of consumption makes it possible to place a sell order, taking into consideration the amount of expected electricity consumption within the home energy network  1  by the time of the actual delivery of cash commodities. 
     (2-4: Configuration of Energy Trade Server  5 ) 
     With reference to  FIG. 13 , functions of the energy trade server  5  according to the present embodiment will be explained.  FIG. 13  is an explanation chart for illustrating a functional configuration of the energy trade server  5  according to the embodiment. 
     As shown in  FIG. 13 , the energy trade server  5  includes a communication unit  51 , an order reception unit  52 , a market price decision unit  53 , a certificate acquisition unit  54 , an energy purchase limitation unit  55 , an energy selling limitation unit  56 , incoming/feeding energy control unit  57 , a local market price collection unit  58 , and a price forecasting unit  59 . 
     (Trading Function) 
     When a user places an order, information of the order is to be input into the order reception unit  52  via the communication unit  51 . The order reception unit  52  receives the order based on the information of the order that has been input, and inputs the order into the market price decision unit  53 . However, the order is not fixed at this stage. The market price decision unit  53  determines a buying price and a selling price (hereinafter, referred to as the market price) based on the balance of supply and demand, depending on the amount of orders that the order reception unit  52  has received. The information on the market price determined by the market price decision unit  53  is to be input into the order reception unit  52 . 
     Note that the trading market may be divided into units of the community energy network  2 . In this case, market prices in each community energy network  2  are to be studied and collected by the local market price collection unit  58 . Subsequently, the information on market prices in each community energy network  2  collected by the local market price collection unit  58  is to be input into the market price decision unit  53 . In this case, the market price decision unit  53  determines the whole market price based on the information on the market prices in each community energy network  2 . Note that the actual deliveries in the electricity trading market of each community energy network  2  may be executed using the community electric storage apparatus  21  (refer to  FIG. 7 ) which has been arranged in each community energy network  2 . 
     As described above, the order reception unit  52  does not make an order fixed at the time of reception of the order from a user. When the order reception unit  52  receives the order, the certificate acquisition unit  54  acquires various certificates from the user before fixing the order. The certificate acquisition unit  54  includes a certificate of space acquisition unit  541 , a certificate of consumption acquisition unit  542 , a certificate of remaining acquisition unit  543 , and a certificate of electric-storage acquisition unit  544 . 
     (Buy Order) 
     If the order received from the user is a buy order, the certificate acquisition unit  54  acquires a certificate of space from the certificate of space acquisition unit  541 , and acquires a certificate of consumption from the certificate of consumption acquisition unit  542 . The certificate of space acquired by the certificate of space acquisition unit  541 , and the certificate of consumption acquired by the certificate of consumption acquisition unit  542  are to be input into the energy purchase limitation unit  55 . The energy purchase limitation unit  55  acquires a public key certificate corresponding to the electric storage apparatus  12  from the certificate authority  4 , and verifies a digital signature contained in the certificate of space and the certificate of consumption using a public key contained in the acquired public key certificate. If the certificate of consumption cannot be acquired, however, the digital signature contained in the certificate of space only will be verified. 
     If successful in the verification, the energy purchase limitation unit  55  reads information on battery space contained in the certificate of space, and information on electricity consumption contained in the certificate of consumption, and inputs them into the order reception unit  52 . Comparing information on the amount of electricity to buy specified by the buy order of the user, and information on remaining battery space and information on electricity consumption that the energy purchase limitation unit  55  has input, the order reception unit  52  fixes the buy order if the amount of electricity to buy is less than (the remaining battery space+electricity consumption). Subsequently, the order reception unit  52  inputs the information on the amount of electricity to buy into the incoming/feeding energy control unit  57 . 
     The incoming/feeding energy control unit  57 , to which the information on the amount of electricity to buy has been input, executes a control instruction so that the electricity for the amount of electricity bought will be supplied to the user who placed the buy order. This control instruction is to be transmitted to the electric storage apparatus  12  of the electricity supplier, the community electric storage apparatus  21 , or a prescribed storage place via the communication unit  51 . The electric storage apparatus  12  of the electricity supplier, the community electric storage apparatus  21 , or a prescribed storage place supplies electricity for the amount of electricity bought to the electric storage apparatus  12  of the user who has placed the buy order. 
     (Sell Order) 
     If the order received by the user is a sell order, the certificate acquisition unit  54  acquires a certificate of remaining from the certificate of remaining acquisition unit  543 , and acquires a certificate of electric-storage from the certificate of electric-storage acquisition unit  544 . The certificate of remaining acquired by the certificate of remaining acquisition unit  543  is to be input into the energy selling limitation unit  56 . The energy selling limitation unit  56  acquires a public key certificate corresponding to the electric storage apparatus  12  from the certificate authority  4 , and verifies a digital signature contained in the certificate of remaining using a public key contained in the acquired public key certificate. If successful in the verification, information on the remaining battery level contained in the certificate of remaining will be input into the order reception unit  52 . 
     The certificate of electric-storage acquisition unit  544  acquires the public key certificate corresponding to the electric storage apparatus  12  from the certificate authority  4 , and verifies a digital signature contained in the acquired certificate of electric-storage using the public key contained in the acquired public key certificate. If succeeded in the verification, information on modified electric-generation capacity contained in the certificate of electric-storage will be input into the order reception unit  52 . As described earlier, the certificate of electric-storage is a certificate that authenticates the electricity is green energy. Therefore, the electricity for the modified electric-generation capacity contained in the certificate of electric-storage is to be bought at the green energy price. If the electricity to sell is not green energy, a certificate of electric-storage will not be acquired. In this case, the certificate of electric-storage acquisition unit  544  inputs a notification that the electricity is not green energy into the order reception unit  52 . Here, however, it is assumed that a certificate of electric-storage can be acquired. 
     When the information on remaining battery level and the information on modified electric-generation capacity have been input, the order reception unit  52  fixes the sell order if the amount of electricity to sell is less than the remaining battery level, comparing information on the amount of electricity to sell specified by the sell order of the user and information on the remaining battery level that the energy selling limitation unit  56  has input. Subsequently, the order reception unit  52  acquires a market price of green energy from the market price decision unit  53 , and fixes the sell order at the market price of green energy for the amount of the modified electric-generation capacity that the certificate of electric-storage acquisition unit  544  has input, out of the amount of electricity to sell specified by the sell order. Further, the order reception unit  52  acquires a market price of green energy from the market price decision unit  53 , and fixes the sell order at the general market price for the amount of electricity to sell. 
     Then, the order reception unit  52  inputs information on the amount of electricity to sell to the incoming/feeding energy control unit  57 . The incoming/feeding energy control unit  57 , to which the information on the amount of electricity to sell has been input, executes a control instruction so that the electricity for the amount of electricity to sell will be transmitted to the prescribed storage place from the electric storage apparatus  12  of the user who has placed the sell order. This control instruction will be transmitted to the electric storage apparatus  12  of the user who has placed the sell order via the communication unit  51 . The electric storage apparatus  12  that has received the control instruction transmits the electricity for the amount of the electricity to sell to the prescribed storage place. 
     (Price Forecast Function) 
     The price forecasting by the energy trade server  5  will be explained. The price forecasting unit  59  includes a market price forecasting unit  591 , a storage unit  592 , and an environmental information collection unit  593 . The market price forecasting unit  591  forecasts future market prices based on changes in the buying and selling prices of electricity determined by the market price decision unit  53  in the past, the current buying and selling prices, changes in the amount of trade in the past, the current amount of trade, and environmental information collected by the later-described environmental information collection unit  593 . 
     The environmental information collection unit  593  implements a method for collecting information regarding environmental factors affecting the amount of electricity by green energy. For example, the environmental information collection unit  593  collects environmental information such as weather forecasts in the past and at present. Moreover, the environmental information collection unit  593  inputs the collected environmental information into the storage unit  592 . The environmental information accumulated in the storage unit  592  is referenced by the market price forecasting unit  591 , and will be used for forecasting of market prices. For example, if the weather has been rainy or cloudy with little sunshine for many days, it may be forecast that the amount of energy supplied by photovoltaic generation will decrease. In this case, the price of green energy is to be forecast higher than usual. 
     The market price predicted in such manner will be transmitted to the interface apparatus  13  of each user and the electric utility company  3  via the communication unit  51 . 
     The function of the energy trade server  5  according to the present embodiment has been explained above. 
     3: Third Embodiment (Services Based on a Green Certificate) 
     Hereinafter, the third embodiment will be explained. Note that structural elements that have functions and structures substantially the same as those of the structural elements according to the first and second embodiments described above are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     (3-1: Configuration of Mobile Device  6 ) 
     With reference to  FIG. 14 , an explanation will be given regarding the configurations of a mobile device  6  and an energy reception apparatus  7  according to the third embodiment of the present invention.  FIG. 14  is an explanatory diagram for illustrating a configuration of a green energy generation system according to the present embodiment. Note that a concrete example of the mobile device  6  is a personal computer, a mobile phone, a portable game machine, a portable information terminal, an information home appliance, a car navigation system, or the like. 
     As shown in  FIG. 14 , the mobile device  6  includes energy consuming parts  60 , a green energy generation apparatus  61 , an electric storage apparatus  62 , an interface apparatus  63 , and a value information acquisition unit  64 . The energy reception apparatus  7  includes an electric energy receiving unit  71 , a signature verification unit  72 , and a value information issuance unit  73 . 
     The energy consuming parts  60  are electric parts for realizing each function of the mobile device  6 . The green energy generation apparatus  61  performs substantially the same function as the green energy generation apparatus  11  according to the first embodiment described above. The green energy generation apparatus  61  is, however, included within an object small enough to be able to mount on the mobile device  6 . 
     The electric storage apparatus  62  is, similar to the green energy generation apparatus  61 , included within an object small enough to be able to mount on the mobile device  6 . Moreover, the electric storage apparatus  62 , different from the electric storage apparatus  12  according to the first embodiment described above, may not issue a certificate of electric-storage. In this case, a certificate of electric-generation will be utilized instead of the certificate of electric-storage. Note that omitting the function to issue the certificate of electric-storage contributes to making the mobile device  6  smaller and much more energy-saving. 
     The interface apparatus  63  performs a method for receiving a user&#39;s operation, and a method for displaying information to the user. The value information acquisition unit  64  acquires value information from the energy reception apparatus  7 . This value information is, for example, a coupon or electric money, or the like. Note that the communication between the mobile device  6  and the energy reception apparatus  7  may be wired or wireless. The energy transmission between the mobile device  6  and the energy reception apparatus  7  may be in a non-contact manner using electromagnetic induction, or in a wired manner using energy lines. 
     When the user operation instructs transmission of electricity from the electric storage apparatus  62  to the energy reception apparatus  7 , the electricity discharged from the electric storage apparatus  62  is to be received by the electric energy receiving unit  71  of the energy reception apparatus  7 . At this time, the certificate of electric-generation that has been issued by the green energy generation apparatus  61  is transmitted to the energy reception apparatus  7  via the interface apparatus  63 . This certificate of electric-generation is to be received by the signature verification unit  72  of the energy reception apparatus  7 . The signature verification unit  72  that has received the certificate of electric-generation acquires a public key certificate corresponding to the green energy generation apparatus  61  from the certificate authority  4 , and verifies a digital signature contained in the acquired certificate of electric-generation using a public key contained in the acquired public key certificate. 
     If successful in the verification, the signature verification unit  72  outputs the certificate of electric-generation externally as well as inputs information on the electric-generation capacity contained in the certificate of electric-generation into the value information issuance unit  73 . The value information issuance unit  73 , to which the information on the electric-generation capacity has been input, transmits the value information corresponding to the electric-generation capacity to the mobile device  6  based on the information on the electric-generation capacity that has been input. The value information transmitted to the mobile device  6  is to be received by the value information acquisition unit  64  of the mobile device  6 , and to be input into the interface apparatus  63 . The interface apparatus  63  stores the value information that has been input by the value information acquisition unit  64 . 
     The configurations of the mobile device  6  and the energy reception apparatus  7  according to the present embodiment have been explained above. 
     (3-2: Configuration of Green Energy Generation Apparatus  61 ) 
     With reference to  FIG. 15  and  FIG. 16 , an explanation will be given regarding the configuration of a green energy generation apparatus  61  according to the present embodiment.  FIG. 15  is an explanatory diagram for illustrating a functional configuration of the green energy generation apparatus  61  according to the present embodiment.  FIG. 16  is an explanatory diagram for illustrating a functional configuration of a certificate of electric-generation issuance unit  614  according to the present embodiment. 
     (Overall Configuration) 
     As shown in  FIG. 15 , the green energy generation apparatus  61  includes a photovoltaic panel  611 , an electric-generation capacity measurement unit  612 , an electric energy transmission unit  613 , a certificate of electric-generation issuance unit  614 , and a communication unit  615 . Note that in the following description, the electric-generation capacity measurement unit  612  and the certificate of electric-generation issuance unit  614  may be collectively called a certificate module. 
     The electric-generation capacity measurement unit  612 , certificate of electric-generation issuance unit  614 , and communication unit  615  may be implemented as software modules. The green energy generation apparatus  61  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the electric-generation capacity measurement unit  612 , certificate of electric-generation issuance unit  614 , and/or communication unit  615  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     The photovoltaic panel  611  performs a method for generating energy by converting the energy of solar light into electricity. The photovoltaic panel  611  utilizes, for example, silicon solar cells or dye-sensitized solar cells or the like. The electricity generated by the photovoltaic panel  611  is to be input into the electric-generation capacity measurement unit  612 . The electric-generation capacity measurement unit  612  measures the electric-generation capacity of the photovoltaic panel  611 . Information on the electric-generation capacity measured by the electric-generation capacity measurement unit  612  is to be input into the certificate of electric-generation issuance unit  614 . 
     The electricity generated by the photovoltaic panel  611  is to be input into the electric energy transmission unit  613  via the electric-generation capacity measurement unit  612 . The electric energy transmission unit  613 , to which the electricity generated by the photovoltaic panel  611  is input, transmits the input electricity to the energy consuming parts  60  or the electric storage apparatus  62 . Note that in the description herein, the electricity output by the electric energy transmission unit  613  is to be transmitted to the electric storage apparatus  62 . 
     The certificate of electric-generation issuance unit  614 , to which the information on the electric-generation capacity has been input from the electric-generation capacity measurement unit  612 , issues a certificate of electric-generation containing the input information on the electric-generation capacity. Note that the detailed functional configuration of the certificate of electric-generation issuance unit  614  will be described later. The certificate of electric-generation that has been issued by the certificate of electric-generation issuance unit  614  is to be input into the communication unit  615 . The communication unit  615 , to which the certificate of electric-generation has been input, transmits the input certificate of electric-generation to the interface apparatus  63 . 
     (Details of the Certificate of Electric-Generation Issuance Unit  614 ) 
     An explanation will be given regarding the detailed functional configuration of the certificate of electric-generation issuance unit  614 . As shown in  FIG. 16 , the certificate of electric-generation issuance unit  614  includes a signature generation unit  6141 , a storage unit  6142 , and a certificate generation unit  6143 . Note that the storage unit  6142  stores a private key and a corresponding public key which have been generated in advance. Further, the public key stored in the storage unit  6142  is assumed to have been certified by the certificate authority  4 . 
     The signature generation unit  6141 , storage unit  6142 , and certificate generation unit  6143  may be implemented as software modules. The green energy generation apparatus  61  or certificate of electric-generation issuance unit  614  may include one or more processors for executing the software modules to perform the functions thereof. Alternatively, the signature generation unit  6141 , storage unit  6142 , and/or certificate generation unit  6143  may comprise one or more specialized hardware modules, such as integrated circuit modules. 
     In the event that the information on the electric-generation capacity is input from the electric-generation capacity measurement unit  612  to the certificate of electric-generation issuance unit  614 , the input information on the electric-generation capacity is to be input into the signature generation unit  6141 . The signature generation unit  6141 , to which the information on the electric-generation capacity has been input, acquires a private key from the storage unit  6142 , and generates a digital signature based on the information on the electric-generation capacity using the private key that has been acquired. The digital signature generated by the signature generation unit  6141  is to be input into the certificate generation unit  6143  along with the information on the electric-generation capacity. The certificate generation unit  6143 , to which the digital signature and the information on the electric-generation capacity have been input, generates a certificate of electric-generation containing the digital signature and the information on the electric-generation capacity which have been input. Subsequently, the certificate of electric-generation generated by the certificate generation unit  6143  is to be input into the communication unit  615 . 
     The configuration of the green energy generation apparatus  61  according to the present embodiment has been explained above. 
     (3-3: Configuration of Electric Storage Apparatus  62 ) 
     With reference to  FIG. 17 , the configuration of the electric storage apparatus  62  according to the present embodiment will be explained.  FIG. 17  is an explanatory diagram for illustrating a functional configuration of the electric storage apparatus  62  according to the present embodiment. 
     (Overall Configuration) 
     As shown in  FIG. 17 , the electric storage apparatus  62  includes an electric energy receiving unit  621 , a charge/discharge control unit  622 , a battery  623 , and an electric energy transmission unit  624 . 
     (At Receiving) 
     At first, the electricity supplied by the green energy generation apparatus  61  is to be received by the electric energy receiving unit  621 . The electricity received by the electric energy receiving unit  621  is to be input into the charge/discharge control unit  622 . The charge/discharge control unit  622 , to which the electricity has been input by the electric energy receiving unit  621 , outputs the electricity to the battery  623  so as to recharge the battery  623 . 
     (At Transmitting) 
     If the battery  623  receives an instruction to discharge via the interface apparatus  63 , the instruction is to be input into the charge/discharge control unit  622 . The charge/discharge control unit  622 , to which the instruction for discharging has been input, discharges the amount of electricity based on the instruction from the battery  623 . The electricity that has been discharged from the battery by the charge/discharge control unit  622  is to be input into the electric energy transmission unit  624 , and is to be transmitted to the energy consuming parts  60  or the energy reception apparatus  7  from the electric energy transmission unit  624 . In this example, however, it is assumed to be transmitted to the energy reception apparatus  7 . 
     The configuration of the electric storage apparatus  62  according to the present embodiment has been explained above. 
     (3-4: Configuration of Interface Apparatus  63 ) 
     With reference to  FIG. 18 , the configuration of the interface apparatus  63  according to the present embodiment will be explained.  FIG. 18  is an explanatory diagram for illustrating a functional configuration of the interface apparatus  63  according to the present embodiment. 
     As shown in  FIG. 18 , the interface apparatus  63  includes a communication unit  631 , a central processing unit  632 , a storage unit  633 , a display unit  634  and an input unit  635 . 
     When the electricity is discharged from the electric storage apparatus  62 , a user inputs information on the amount of discharged electricity or the like using the input unit  635 . The information that has been input using the input unit  635  is to be input into the electric storage apparatus  62  via the central processing unit  632  and the communication unit  631 . When the process of discharging electricity is performed by the electric storage apparatus  62 , the communication unit  631  receives a certificate of electric-generation from the green energy generation apparatus  61 . The certificate of electric-generation received by the communication unit  631  is to be input into the central processing unit  632 . The central processing unit  632 , to which the certificate of electric-generation has been input, transmits the input certificate of electric-generation to the energy reception apparatus  7  via the communication unit  631 . 
     Further, when the user inputs an instruction to discharge using the input unit  635 , the central processing unit  632  transmits a discharging instruction and information on the amount of electricity to discharge to the electric storage apparatus  62  via the communication unit  631 . Note that the storage unit  633  is used to store value information received from the energy reception apparatus  7 , or information that a user has input, or is used to store a public key or the like corresponding to the green energy generation apparatus  61  as needed. The storage unit  633  is used to store a program that defines operations of the central processing unit  632 . 
     The configuration of the interface apparatus  63  according to the present embodiment has been explained above. 
     (3-5: Operation for Transmitting Electric Energy) 
     Next, with reference to  FIG. 19  and  FIG. 20 , the method for transmitting electric energy will be explained.  FIG. 19  is an explanatory diagram for illustrating a flow of operation for transmitting electric energy according to the present embodiment.  FIG. 20  is an explanatory diagram for illustrating an example of a graphical interface to be displayed in the flow of operation for transmitting electric energy according to the present embodiment. 
     As shown in  FIG. 19 , the mobile device  6  and the energy reception apparatus  7  execute a mutual authentication (S 101 ). If successful in the mutual authentication, the mobile device  6  notifies the energy reception apparatus  7  of the maximum amount of electric energy transmission (S 102 ). This maximum amount of electric energy transmission is the remaining battery level of the battery  623 , the remaining level calculated by subtracting a certain margin necessary for the operation of the mobile device  6  from the remaining battery level, or the maximum value of the amount of energy transmission which has been input by a user. The energy reception apparatus  7  encourages the user to select the amount of energy transmission and allows the user to select the amount of energy transmission (S 103 ), as shown in  FIG. 20 . 
     If the electric-generation capacity has been selected by the user, the energy reception apparatus  7  notifies the mobile device  6  of the electric-generation capacity selected by the user, and requests the transfer of a certificate of electric-generation as well (S 104 ). Subsequently, the mobile device  6  encourages the user to select whether to allow energy transmission, as shown in  FIG. 2  (S 105 ). If the user allows the energy transmission, the mobile device  6  starts to transmit electricity, and transmits (assigns) the certificate of electric-generation to the energy reception apparatus  7  as well (S 106 ). Note that granting value information will be done after the step S 106  in accordance with the electric-generation capacity that is authenticated by the certificate of electric-generation. 
     The method for transmitting electric energy according to the present embodiment has been explained above. 
     &lt;4: Example of Hardware Configuration&gt; 
     The functions of the various types of certificate issuance units, interface apparatuses, mobile devices, energy trade servers, or the like described above can be realized by using the hardware configuration of an information processing apparatus shown in  FIG. 21 , for example. That is, the function of each structural element is realized by controlling the hardware shown in  FIG. 21  by using a computer program. Additionally, the mode of this hardware is arbitrary, and may be a personal computer, a mobile information terminal such as a mobile phone, a PHS or a PDA, a game machine, or various types of information appliances. Moreover, the PHS is an abbreviation for Personal Handy-phone System. Also, the PDA is an abbreviation for Personal Digital Assistant. 
     As shown in  FIG. 21 , this hardware mainly includes a CPU  902 , a ROM  904 , a RAM  906 , a host bus  908 , and a bridge  910 . Furthermore, this hardware includes an external bus  912 , an interface  914 , an input unit  916 , an output unit  918 , a storage unit  920 , a drive  922 , a connection port  924 , and a communication unit  926 . Moreover, the CPU is an abbreviation for Central Processing Unit. Also, the ROM is an abbreviation for Read Only Memory. Furthermore, the RAM is an abbreviation for Random Access Memory. 
     The CPU  902  functions as an arithmetic processing unit or a control unit, for example, and controls an entire operation or a part of the operation of each structural element based on various programs recorded on the ROM  904 , the RAM  906 , the storage unit  920 , or a removable recording medium  928 . The ROM  904  is means for storing, for example, a program to be loaded on the CPU  902  or data or the like used in an arithmetic operation. The RAM  906  temporarily or perpetually stores, for example, a program to be loaded on the CPU  902  or various parameters or the like arbitrarily changed in execution of the program. 
     These structural elements are connected to each other by, for example, the host bus  908  capable of performing high-speed data transmission. For its part, the host bus  908  is connected through the bridge  910  to the external bus  912  whose data transmission speed is relatively low, for example. Furthermore, the input unit  916  is, for example, a mouse, a keyboard, a touch panel, a button, a switch, or a lever. Also, the input unit  916  may be a remote control that can transmit a control signal by using an infrared ray or other radio waves. 
     The output unit  918  is, for example, a display device such as a CRT, an LCD, a PDP or an ELD, an audio output device such as a speaker or headphones, a printer, a mobile phone, or a facsimile, that can visually or aurally notify a user of acquired information. Moreover, the CRT is an abbreviation for Cathode Ray Tube. The LCD is an abbreviation for Liquid Crystal Display. The PDP is an abbreviation for Plasma Display Panel. Also, the ELD is an abbreviation for Electro-Luminescence Display. 
     The storage unit  920  is a device for storing various data. The storage unit  920  is, for example, a magnetic storage device such as a hard disk drive (HDD), a semiconductor storage device, an optical storage device, or a magneto-optical storage device. The HDD is an abbreviation for Hard Disk Drive. 
     The drive  922  is a device that reads information recorded on the removal recording medium  928  such as a magnetic disk, an optical disk, a magneto-optical disk, or a semiconductor memory, or writes information in the removal recording medium  928 . The removal recording medium  928  is, for example, a DVD medium, a Blu-ray medium, an HD-DVD medium, various types of semiconductor storage media, or the like. Of course, the removal recording medium  928  may be, for example, an IC card on which a non-contact IC chip is mounted or an electronic device. The IC is an abbreviation for Integrated Circuit. 
     The connection unit  924  is a port such as an USB port, an IEEE1394 port, a SCSI, an RS-232C port, or a port for connecting an externally connected device  930  such as an optical audio terminal. The externally connected device  930  is, for example, a printer, a mobile music player, a digital camera, a digital video camera, or an IC recorder. Moreover, the USB is an abbreviation for Universal Serial Bus. Also, the SCSI is an abbreviation for Small Computer System Interface. 
     The communication unit  926  is a communication device to be connected to a network  932 , and is, for example, a communication card for a wired or wireless LAN, Bluetooth (registered trademark), or WUSB, an optical communication router, an ADSL router, or various communication modems. The network  932  connected to the communication unit  926  is configured from a wire-connected or wirelessly connected network, and is the Internet, a home-use LAN, infrared communication, visible light communication, broadcasting, or satellite communication, for example. Moreover, the LAN is an abbreviation for Local Area Network. Also, the WUSB is an abbreviation for Wireless USB. Furthermore, the ADSL is an abbreviation for Asymmetric Digital Subscriber Line. 
     The example of a hardware configuration capable of realizing functions of the interface apparatus, the mobile device, the energy trade server, or the like according to the present embodiment has been explained above. 
     &lt;5: Conclusion&gt; 
     Lastly, the technical contents according to the embodiments will be briefly described. The technical contents stated here can be part of various apparatuses, such as a PC, a mobile phone, a portable game machine, a portable information terminal, an information appliance, a car navigation system, and the like. For example, a green energy generation apparatus described below can be mounted on such apparatuses. 
     The green energy generation apparatus according to the present embodiment includes a green energy generator, an electric-generation capacity measurement unit, and a certificate of electric-generation issuance unit, which are like the following. The green energy generation apparatus generates electricity using renewable energy or environmentally friendly resources. The above electric-generation capacity measurement unit measures the electric-generation capacity generated by the green electric energy generation unit. Further, the above certificate of electric-generation issuance unit acquires electric-generation capacity information indicating the electric-generation capacity measured by the electric-generation capacity measurement unit, and issues a certificate of electric-generation that authenticates the validity of the acquired electric-generation capacity information. 
     By using the certificate of electric-generation issued in such a manner, it may be possible to prove the electric-generation capacity generated by green electric energy generation to other people. For example, it may be possible to develop a structure in which electricity can be bought at a green energy price when selling electricity along with a green certificate. For another example, it may be possible to trade electricity at a price for green energy by trading any arbitrary electricity with the green certificate. 
     Thus, if green certificate changes the trading price, the green certificate will have monetary value. If the green certificate has the monetary value, the green certificate can be treated as a marketable security. As a result, the value of green generation becomes apparent electric energy consumers, and contributes the spread of the green electric energy generation apparatus. Needless to say, when the green energy generation apparatus becomes widespread and replaces electric energy generation using fossil fuels, the green energy generation will hinder the decline of the global environment, even if only a little. 
     An embodiment has been explained in detail above with reference to the attached drawings. However, the present invention is not limited to this example. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
     Photovoltaic generation has been described as an example of green electric energy generation in the above explanation; however, the technology according to the present embodiment can be applied to various types of generation, such as wind energy generation, geothermal energy generation, hydro energy generation, atomic energy generation, biomass energy generation, natural gas generation, fuel cell generation, and the like. A battery has been described as an example of method of charging electric energy. Specifically, the technology according to the present embodiment can be applied in the same way to various methods of charging electric energy, such as a secondary battery (Li-Ion battery, NaS battery, or the like), a capacitor (an electric double-layer capacitor, or the like), a lift storage system, or the like. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-026563 filed in the Japan Patent Office on Feb. 9, 2010, the entire content of which is hereby incorporated by reference. 
     Reference Signs List 
       1  Home energy network 
       10  Energy consuming device 
       11  Green energy generation apparatus 
       111  Photovoltaic panel 
       112  Electric-generation capacity measurement unit 
       113  Electric energy transmission unit 
       114  Certificate of consumption issuance unit 
       1141  Signature generation unit 
       1142  Storage unit 
       1143  Certificate of consumption issuance unit 
       115  Communication unit 
       12  Electric storage apparatus 
       121  Electric energy receiving unit 
       122  Charge/discharge control unit 
       123  Battery 
       124  Electric energy transmission unit 
       125  Certificate of electric-storage issuance unit  125   
       1251  Signature verification unit 
       1252  Electric-generation capacity modification unit 
       1253  Signature generation unit 
       1254  Storage unit 
       1255  Certificate generation unit 
       126  Communication unit 
       127  Certificate of remaining issuance unit 
       1271  Remaining acquisition unit 
       1272  Signature generation unit 
       1273  Storage unit 
       1274  Certificate generation unit 
       128  Certificate of space issuance unit 
       1281  Space acquisition unit 
       1282  Signature generation unit 
       1283  Storage unit 
       1284  Certificate generation unit 
       13  Interface apparatus 
       131  Communication unit 
       132  Central processing unit 
       133  Storage unit 
       134  Display unit 
       135  Input unit 
       136  Certificate of consumption issuance unit 
       1361  Consumption setting unit 
       1362  Signature generation unit 
       1363  Storage unit 
       1364  Certificate generation unit 
       2  Community energy network 
       20  Energy network 
       21  Community electric storage apparatus 
       3  Electric utility company 
       4  Certificate authority 
       5  Energy trade server 
       51  Communication unit 
       52  Order reception unit 
       53  Market price decision unit 
       54  Certificate acquisition unit 
       541  Certificate of space acquisition unit 
       542  Consumption certificate acquisition unit 
       543  Certificate of remaining acquisition unit 
       544  Certificate of electric-storage acquisition unit 
       55  Energy purchase limitation unit 
       56  Energy selling limitation unit 
       57  Incoming/feeding energy control unit 
       58  Local market price collection unit 
       59  Price forecasting unit 
       591  Market price forecasting unit 
       592  Storage unit 
       593  Environmental information collection unit 
       6  Mobile device 
       60  Energy consuming parts 
       61  Green energy generation apparatus 
       611  Photovoltaic panel 
       612  Electric-generation capacity measurement unit 
       613  Electric energy transmission unit 
       614  Certificate of electric-generation issuance unit 
       6141  Signature generation unit 
       6142  Storage unit 
       6143  Certificate generation unit 
       615  Communication unit 
       62  Electric storage apparatus 
       621  Electric energy receiving unit 
       622  Charge/discharge control unit 
       623  Battery 
       624  Electric energy transmission unit 
       63  Interface apparatus 
       631  Communication unit 
       632  Central processing unit 
       633  Storage unit 
       634  Display unit 
       635  Input unit 
       64  Value information acquisition unit 
       7  Energy reception apparatus 
       71  Electric energy receiving unit 
       72  Signature verification unit 
       73  Value information issuance unit