Abstract:
The invention provides a plant for production of energy, comprising any type of heat or energy source including but not limited to solar power sources, nuclear reactors, fossil fuel plants, wind power plants, tidal power plants, waste heat power plants and geothermal sources, operatively arranged at an input side of the plant, and heat delivery or energy production means such as turbine-electric generator sets, operatively arranged at a delivery side of the plant. The plant is distinctive in that it further comprises a thermal energy storage with integrated heat exchanger, comprising a solid state thermal storage material, a heat transfer fluid and means for energy input and output, wherein: the storage comprises at least one heat transfer container, solid state thermal storage material is arranged around the heat transfer container, the heat transfer container contains the heat transfer fluid and the means for energy input and output, so that all heat transferring convection and conduction by the heat transfer fluid takes place within the respective heat transfer container, the thermal energy storage with heat exchanger has been arranged inside thermal insulation, and the solid state thermal energy storage with heat exchanger, has been arranged between the input side and delivery side of the plant for storage and heat exchange of thermal energy, the storage is coupled directly or via an additional heat exchanger to the source and the storage is coupled directly or via an additional heat exchanger to the delivery side of the plant.

Description:
FIELD OF THE INVENTION 
     The present invention relates to plants for production of energy from sources like solar power, particularly concentrated solar power (CSP) plants, but also from other thermal energy sources including but not limited to nuclear reactors, fossil fuel plants and deep earth geothermal sources. More specifically, the invention relates to modification of existing plant designs or for building simpler and more effective new plants. 
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     Significant research and development efforts are being made on an international scale to improve the efficiency and environmental performance for energy producing systems. Today about half of all such efforts are made within the renewable energy sector rather than for traditional fossil or nuclear fuel type thermal power production. It has become clear that energy storage will be a key technology for making further advances, and large investments are currently being made in developing such capabilities, notably by storing energy using rechargeable batteries, pumped hydro storage, compressed air, flywheels, conversion to hydrogen, and heat storage including heat storage with material phase change. 
     It is generally recognized that energy storage can facilitate time dependent adaptation of power delivery to consumers and the market in general, and it can provide security of delivery by way of bridging power delivery when the primary power production is insufficient or it fails. 
     For some types of renewable energy production having storage is absolutely necessary. One such field is concentrated solar power (CSP) where heat storage is used to compensate for insufficient or failing heat production, such as during day time when cloud cover occurs and during the night hours when there is no sun. In the case of CSP using parabolic trough collectors the heat is generated by reflected and focused sun rays heating oil in a pipe system; this oil is thereafter heat exchanged with molten salt which is stored in large, insulated storage tanks. The oil is a feasible mineral, organic or synthetic heat transfer oil, such as Therminol. When stored heat is needed for supplementary or extended energy production it is extracted by a reverse heat exchange between molten salt and oil. Thereafter the oil is once more heat exchanged into water-steam which in turn is used to produce electricity via turbines and electric generators. This technology has some severe disadvantages in that it involves using very expensive storage fluids such as molten salt, it requires multiple oil/salt heat exchangers, it needs at least two large molten salt storage tanks, it requires molten salt pumps and corresponding pumping energy to move the molten salt to and from the tanks, and there is a risk of the salt solidifying in pipes or other structures which can occur even at very high temperature. 
     Another version of CSP is where a large field of mirrors (heliostats) reflects sun rays onto a high tower where a receiver filled with a high temperature working fluid, such as molten salt, is used for photon-heat conversion. In the case of using molten salt as working fluid this fluid may be directly heat exchanged with water-steam to generate electric power using turbines and generators. Alternatively, the molten salt may be stored in large tanks for later to be used to generate electricity as explained above. 
     Recent research efforts also consider using steam as working fluid for the CSP tower technology; in this way heat exchangers from molten salt to steam may be avoided. The problem with direct steam technology is to be able to store the heat for delayed use, particularly combined with producing electricity effectively. The current invention provides a possible solution to this problem. 
     There are numerous traditional technologies where the main source of energy is heat and pressure; such as fossil fuel (coal, oil, gas) plants and nuclear power plants. Although such plants may not depend on having storage for periodical lack of energy production, heat storage may be a great advantage and economically profitable for such plants as well. For instance, heat storage may facilitate full use or better use of the heat production capability throughout a 24 hour day cycle since heat produced during the night may be fully or partly stored and provide higher energy production during the day time in accordance with market demands. Moreover, storage is of great value for providing continuity and security of energy supply or for dealing with temporal bottlenecks in the electric grid system. 
     The objective of the present invention is to provide a plant for production of energy, which plant is beneficial over the previous technology with respect to issues mentioned. Further, the plant storage should preferably:
         Be able to simplify the overall process of heat accumulation, transportation and storage as compared with existing systems   Be able to operate with temperatures and pressures most suitable for the heat collection and transportation of heat to the storage   Be able to accommodate different types of working fluid deemed most suitable for the above mentioned operation   Be able to efficiently transport heat out of storage by way of fluids with temperature and pressure suitable for the heat extraction process   Be able to accommodate types of working fluids most suitable for the heat delivery from storage   Be able to operate in a primarily heat exchanger mode where the heat in the working fluid of the primary heat input pipe loop is heat exchanged directly and simultaneously within the storage heat exchangers with another type of working fluid in the heat extraction pipe system   Be suitable for implementation with already commercially available components such as pumps, valves, pipes, sensors, and control systems   Provide cost and efficiency advantages over existing systems   Be environmentally safe   Be easy to integrate within and modify existing facilities as well as for being used in design and operation of new facilities       

     SUMMARY OF THE INVENTION 
     The invention provides a plant for production of energy, comprising any type of heat or energy source including but not limited to solar power sources, nuclear reactors, fossil fuel plants, wind power plants, tidal power plants, waste heat power plants and geothermal sources, operatively arranged at an input side of the plant, and heat delivery or energy production means such as turbine-electric generator sets, operatively arranged at a delivery side of the plant. The plant is distinctive in that the plant further comprises a thermal energy storage with integrated heat exchanger, comprising a solid state thermal storage material, a heat transfer fluid and means for energy input and output, wherein: 
     the storage comprises at least one heat transfer container, 
     solid state thermal storage material is arranged around the heat transfer container, 
     the heat transfer container contains the heat transfer fluid and the means for energy input and output, so that all heat transferring convection and conduction by the heat transfer fluid takes place within the respective heat transfer container, 
     the thermal energy storage with heat exchanger has been arranged inside thermal insulation, and 
     the solid state thermal energy storage with heat exchanger, has been arranged between the input side and delivery side of the plant for storage and heat exchange of thermal energy, the storage is coupled directly or via an additional heat exchanger to the source and the storage is coupled directly or via an additional heat exchanger to the delivery side of the plant. 
     The term coupled directly in this context means that the working fluid in the source or the delivery side is the same as that in the storage and the systems are coupled merely via piping, valves, pumps or compressors, without additional heat exchangers or storages. 
     Preferable embodiments of the plant are defined in the dependent claims, to which reference is made. 
     The thermal energy storage and heat exchanger is termed a NEST thermal energy storage with heat exchanger. 
     Most preferably, the NEST thermal energy storage and heat exchanger comprises one or more heat transfer containers arranged vertically standing side by side, inside an outer container or pipe section, the space between the one or more heat transfer containers and the outer container or pipe section has been filled with concrete or other solid state materials. No traditional armouring bars or structures will be required, facilitating the production of the heat storage of the plant. By arranging many cylindrical outer containers or pipe section vertically standing side by side, each filled with concrete and one or more heat transfer containers as described above, and coupling the heat transfer container means for heat input and output together and to sources and the delivery side as described and illustrated below, large plants can be provided surprisingly easy. The pipe in pipe, or container in container solution as mentioned above, with concrete between the heat transfer containers(s) and the outer pipe or container, provides effective basic units or heat cells for heat storage of a plant of the invention. Connecting pairs of heat transfer containers at the bottom, thereby enabling the working fluid to flow from one to the other, with concrete between the heat transfer containers and the outer pipe or container, provides another effective basic unit or heat cell for heat storage of a plant of the invention. In another preferred embodiment the solid state material of the heat cells of the heat storage comprises grouting and concrete, the grouting is arranged between the concrete and the heat transfer containers. The storage, comprising many basic cells or units, comprises insulation around the storage, preventing heat exchange with and heat loss to the ground or the air. 
     Please refer to U.S. Pat. No. 332,707 or patent application PCT/NO2012/050088, herein incorporated by reference, for a detailed description of the NEST solid state thermal storage per se and particularly the preferred embodiments thereof, some of which are used in the plants of the invention. 
     The present invention is beneficial with respect to all of the above mentioned issues, as will be clear from the further description and accompanying figures. 
     The primary heat input system is in the most typical case a closed loop of pipes filled with working fluid where cold fluid is pumped through the heat generating system where it is heated by some energy source and thereafter transported into the storage where heat is delivered and, hence, the temperature and pressure of the fluid will decrease. In the case of concentrated solar power (CSP) heating comes from sun light (photon radiation) being reflected onto a receiver filled with circulating working fluid. A main type of sun energy receiver is parabolic troughs in which parabolic reflectors (mirrors) reflect sun light onto a pipe receiver in the focal point in which the working fluid flows and is heated. Another type is heliostat field reflectors (mirrors) reflecting sun light onto a tower with a receiver field on top through which the working fluid is circulated and heated. Other types are parabolic dish systems where the sun rays are reflected and focused onto a local receiver attached to the movable mirror and within the receiver the working fluid is being heated. Yet another type of CSP system under development is based on Fresnel collectors. For some solar heating systems it can be beneficial to operate with a lower temperature of the incoming working fluid and thereby achieve better energy absorption and overall efficiency. In current CSP systems the working fluid in the primary system is typically thermal oil that is heat exchanged with molten salt in a separate heat exchanger before being stored as molten salt in large tanks. Alternatively, as in CSP tower technology, the primary working fluid may be molten salt that can go directly into tanks for heat storage. 
     A drawback by current molten salt storage systems is that the temperature of the working fluids in the heat transporting systems exiting the heat exchanger has to be safely higher than the solidification temperature of the molten salt, typically more than 260 degrees C. for the binary nitrate salt mix used today, often referred to as Solar Salt. This severely limits the use of molten salt as heat storage in applications with low or moderate temperature levels. Other salt mixes may provide a lower temperature limit than Solar Salt. If the working fluid is oil the maximum and minimum temperatures in this fluid will roughly speaking have to be in the range between 400 (boiling of oil) and 300 degrees C. (solidification of molten salt). The fact that the dynamic temperature range can only be about 100 degrees for such systems greatly reduces their efficiency. According to the current invention the heat storage is mainly of solid type and the heat exchange takes place in the heat exchangers that are integrated within the storage itself. This implies that there will be no lower limit for temperature in the storage other than what is acceptable for the working fluids themselves. 
     This not only implies a potential for better utilization of the storage itself due to higher dynamic storage temperatures, but may also increase efficiency of the solar energy absorption system. 
     Some simple type solar heating systems are only used for heating water and cannot be used to produce electricity. However, CSP systems are currently being developed that can generate high pressurize steam that can be used directly for running turbines. In particular a target is to develop systems that generate supercritical fluid where there is no distinct difference between water and steam. For instance, by operating with temperatures in the range 550° to 600° C. or higher under supercritical pressure one may achieve much improved turbine efficiency compared with steam turbines operating at subcritical pressure. For CSP use of storage is a key technology to compensate for variable solar energy influx depending on time and variable cloud cover and no heating during dark hours, and also for efficiency. Also in this setting the current invention offers a good and efficient solution to the storage problem. When heat should be stored the pressurized steam is fully or partly transferred to the storage which, by way of its heat exchangers, transfers the heat to the solid storage material. When pressurized steam is wanted for the turbines heat is simply taken from the storage by way of steam using the same heat exchangers and directed into the turbine loop. 
     There are many other types of sources of heat that can be dealt with in a similar way. One such setting is pressurized steam from geothermal reservoirs or from active geothermal or upwelling mantle zones. Typically geothermal installations produce heat by the same rate during night and day. By storing heat during the night and by tapping this heat from the storage during the day one will be able to increase the power production during the peak hours of the day far beyond what comes from the constant flow of steam from the geothermal reservoir. Implementation of this concept, which is a typical feature of embodiments of the invention, of course requires additional turbine capacity to increase the power production. Fossil fuel plants, nuclear plants and waste heat power plants are other possible sources delivering heat energy to a plant of the invention. Use of waste heat from the industry is yet another example. Wind power plants, tidal power plants, and silicon wafer based solar power plants are examples of sources delivering electricity to a plant of the invention. Sources delivering electricity require either an external heating element that uses electricity to heat the working fluid before it enters the storage, or heating elements integrated in the heat exchangers, as the means for heat input in the heat storage of the plant, for converting electricity to heat. 
     Previously mentioned solid storage invention also describes how electricity may be used as heat source by way of Joule type electric heating elements that are directly inserted into the heat exchanger containers. Please refer to patent application PCT/NO2012/050088, for a detailed description of the solid state thermal storage per se. 
     A key trait of the current invention is its flexibility with regard to adaptation to different types of working fluids or heat transfer fluids. For instance, the storage heating loop may be based on thermal oil and the heat extraction for turbine loop may be based on water-steam. Another alternative is that the storage heating loop is based on water to steam and also the heat extraction loop from storage is based on from water to steam. These concepts will be explained in further detail later. It is to be noted that the current invention can work with steam under very high pressures, such as supercritical steam, because the steam goes in adequately dimensioned pipes that can sustain such conditions and that such expanding pipes do not represent any problem for the type of solid storage which is a part of the plant of the invention. 
     The current invention may also be used with working fluids other than oil and water-steam. Other fluids include, but are not limited to, sub-saturated or saturated water, molten salt, synthetic molten salts, liquid metals and alloys, various types of composite fluids, particles suspended in fluids, gases, etc. 
     The innovation may have many other forms and usages as well. For instance it can be implemented to complement or replace salt storage in already existing CSP plants with salt storage. In such case the working fluid for heat storage as well as the working fluid for heat extraction from the storage will typically be oil. As will be shown in later this situation may well be dealt with by the current invention. 
     A special form of usage of the invention is when it is operated as primarily a heat exchanger from one type of working fluid to another type of working fluid rather than being used in heat storage or heat extraction modes. The most typical situation may be when hot oil from the primary heating loop is heat exchanged directly with water to steam within the heat exchangers of the storage. This application may be of value for CSP installations when operating during the day. The purpose of this concept is that one may be able to fully avoid having a separate heat exchanger (boiler) for going from heated oil to steam and thereby achieve considerable cost savings. Truly, some heat will leak into the solid state storage when the storage heat is being operated primarily in a heat exchanger mode; however, such heat is not lost but will rather be available for later use when the storage is being operated in a heat extraction mode. 
    
    
     
       FIGURES 
       The invention is illustrated by 10 figures, of which: 
         FIG. 1  shows a process diagram which illustrates a prior art typical CSP installation with salt storage or similar, and illustrates which parts thereof can be replaced and improved by the current invention 
         FIG. 2  shows a process diagram for a plant of the invention, wherein the heat storing mode of a CSP installation or similar where the molten salt storage has been replaced with a solid state storage of the current invention and where the same working fluid is used from heat storage as for heat extraction 
         FIG. 3  shows a process diagram for the heat extraction mode of a CSP installation or similar of the invention, where the molten salt storage has been replaced with a solid state storage and where the same working fluid is used from heat storage as for heat extraction for subsequent energy use 
         FIG. 4  shows a heat exchanger for the solid state storage where the same working fluid is used for heat storage as for heat extraction from the storage. This version also has an alternative and simpler design which will be found in the detailed description. 
         FIG. 5  shows a heat exchanger for the solid state storage where the same working fluid is used for heat storage as for heat extraction from the storage and where the heat exchange fluid within the storage heat exchangers are also of the same type 
         FIG. 6  shows a process diagram for the heat storage mode as well as for the heat extraction mode of a CSP installation or similar of the invention where one type of working fluid is used for heat storage into the solid state storage and another type of working fluid such as water-steam is used for the heat extraction and subsequent energy use 
         FIG. 7  shows a heat exchanger for the solid state storage where one type of working fluid is used for heat storage and another type of working fluid is used for heat extraction from the storage 
         FIG. 8  shows a heat exchanger for the solid state storage where one type of working fluid is used for heat storage as well as for the heat exchanger fluid in the solid state storage and another type of working fluid is used for heat extraction from the storage 
         FIG. 9  shows a process diagram for the heat storing mode of a CSP installation or similar plant of the invention, where the heat generating unit produces steam which is used directly to carry heat to the solid state storage 
         FIG. 10  shows a process diagram for the heat extraction mode of a CSP installation or similar plant of the invention, where the heat generating unit produces steam which is used directly to carry heat to the solid state storage and where water-steam is also used for the heat extraction and subsequent energy use. 
         FIGS. 11-14  illustrate embodiments of the previously mentioned pipe in pipe solution, for which the at least one heat transfer container is the inner pipe and the volume between the heat transfer container, sometimes termed heat pipe, has been filled with concrete. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates one type of target application of the invention; this figure shows a schematic process diagram for a prior art parabolic, trough type, concentrated solar power installation not according to the invention. A purpose of this figure is to illustrate the complexity of such conventional CSP plants and to show how the system can be greatly simplified and major parts of the system can be replaced by the current invention. In heat storing mode trough type parabolic mirrors  1  heat up oil in receiver  2  through which oil is pumped into pipe  11  into a valve  51  and from there into pipe  12 , after which the oil goes into a heat exchanger  41  to deliver heat, after this the cooled oil is pumped by pump  71  through pipes  13  and  14  back into the trough heat absorber  2  for renewed heating. This pipe loop represents the primary heating loop; the working fluid in such a loop is typically thermal oil that can sustain high temperature whereas other working fluids may also be considered. The heat provided by the primary loop is heat exchanged with molten salt which is sent from a “cold” storage tank  4  by way of pump  73  through pipe  16 , heat exchanger  41  and pipe  15  into a “hot” molten salt tank  3 . 
     Later, during heat extraction mode, hot molten salt is sent from tank  3  by way of pump  72  through pipe  15 , heat exchanger  41  and pipe  16  back into the cold storage tank  4 . During this heat delivery process oil is circulated and heated by the molten salt in the heat exchanger  41  and sent via pipe  12 , valve  51  and pipe  17  into another heat exchanger  42 . After delivering heat in heat exchanger  42  the oil is by pump  74  pumped back through pipe  18 , valve  52 , and pipe  13  back into the heat exchanger  41  for renewed heating of the thermal oil in this pipe loop. 
     Water under high pressure is heated and converted to steam in heat exchanger (boiler)  42  and goes through pipe  19  into turbine-generator system  81  for generation of electricity and further through pipe  20  into a cooling exchanger system (condenser)  43  utilizing a cooling tower  82  or other cooling system, after which the recycled turbine cycle water (feedwater) is pumped to high pressure by pump  75  via pipe  21  back into heat exchanger  42  to complete the cycle. 
     It seems clear that the system described in  FIG. 1  is rather complicated and it is also very expensive. Molten salt has very good heat storing capacity, but molten salt suitable for such applications is also very expensive. A serious drawback is also that molten salt cannot be used in lower temperature systems as the salt will solidify. 
     The system diagram  FIG. 1  further shows an area that is defined by a dotted line  90  and a dotted line  91 . The current invention can in principle replace all the components inside dotted line  90 , apart from pump  74 , and in general replace all components within dotted line  91 ; in both cases achieving a much simpler and more cost efficient plant or system. 
       FIG. 2  shows a system flow diagram for one type of implementation of the plant of the current invention, with simplifications compared with frame  91  in  FIG. 1 . What is shown in this figure represents a modification of what is shown in  FIG. 1  with the difference that the salt storage tanks and the oil/salt heat exchangers are replaced by a solid state storage of the type associated with the invention. To clarify matters  FIG. 2  indicates the oil flow only for the primary loop during heat storing operation. Oil is heated in solar heater  2  and sent via pipe  22  and valve  53  into solid state storage  100  with heat exchangers  101  where it delivers heat and is cooled down. It is thereafter sent via pipe  24 , valve  54 , pipe  25  and pump  76  back into the solar heating system. 
       FIG. 3  shows the same system as in  FIG. 2  when it is operating in a heat extraction mode. Cooled oil is sent through the solid state heat storage and the heat exchangers  101  and further on via pipe  23 , valve  53 , and pipe  26  into heat exchanger (boiler)  42  where it is cooled and sent back to the storage via pipe  27 , pump  77 , valve  54  and pipe  24  back into the storage for reheating. What takes place in the boiler and the turbine loop  19 ,  81 ,  20 ,  43 ,  21  and  75  is in principle exactly the same as has been explained for salt storage technology in connection with  FIG. 1 . 
     By comparing the system in  FIGS. 2 and 3  with the salt storage base case it is seen that the heat exchanger between oil and salt is no longer necessary, and the two large tanks for molten salt has been replaced with one solid state storage. Equally important, the lower storage operational temperature associated with risk for solidification of the molten salt is no longer applicable. This means that the oil working fluid, as well as the solid state storage, can operate with a much higher dynamic temperature range and/or at lower temperatures, simply because the lower temperature bound imposed by the molten salt is no longer there. This can also have positive implications for the efficiency of the solar energy catching system  1  and  2 . 
     The fact is that the same type of working fluid is used for delivery of heat to the solid state storage as for extracting heat from it. In such case the heat exchangers of current type can be rather simple.  FIG. 4  shows a pipe loop within a storage stack in accordance with one of the embodiments of patent PCT/NO2012/050088. For a detailed description of the solid state thermal storage per se, please refer to said patent applications. The working fluid  111  goes in a pipe  101  in a loop inside a vertical heat exchanger container  120  filled with heat transfer fluid  112 . Next to the heat exchange container  120  and in full contact with the container there are solid state heat storage materials  121 , illustrated out of scale for clarity, which can be composed of zones with different material properties. In heat storage mode the working fluid is warmer than the heat exchange or transfer fluid  112  which in turn is warmer than the solid state heat storage material  121 . In the heat extraction mode the temperature situation is the opposite. Note that the working fluid  111  may have high pressure whereas the heat exchange fluid  112  in the container  20  may have low or near atmospheric pressure and thus does not exert any pressure onto the solid state materials. In a typical case of the configuration shown in  FIGS. 2 and 3  the working fluid used in the primary loop as well as for heat transfer fluid in the heat exchanger will be thermal oil; however, other types of working fluids may also be considered. An even simpler alternative of  FIG. 4 , which can be used when the working fluid  111  is the same as the heat exchanger fluid  112 , is to replace the heat transfer fluid  112  with solid state heat storage material  121 . What was prior the heat exchange container  120  now becomes a cylindrical, or any other suitable geometry such as superelliptic, rectangular etc, casting form which subsequently also acts as an outer reinforcement shell. This alternative can have one or more pairs of vertical heat transfer containers  101  connected at the bottom in which the working fluid/heat transfer fluid  111  flows. In this configuration heat is delivered to and from the solid state storage media largely by convection. This version of  FIG. 4  is shown in  FIG. 13  and  FIG. 14 , for the special case of having two pairs of vertical heat exchangers. Another simple design is shown in  FIG. 5 . This version of heat exchanger can be used when the working fluid  111  is the same as the heat exchanger fluid  112 . As seen from the figure the working fluid is led through a pipe into the heat exchange container and delivers heat largely by convection before being pressed out with a lower temperature. In the heat extraction mode cold working fluid is pressed into the container where it absorbs heat and comes out with a higher temperature. 
       FIGS. 4 and 5  illustrate basic principles for types of heat exchangers that may be used. In a real size storage there may be very large number of heat exchangers coupled by means of pipes and valves. The flow through these heat exchangers may be by way of serial as well as parallel coupling. 
       FIG. 6  shows a system flow diagram for another type of plant implementation of the current invention. In this case one type of working fluid is used in the primary heating loop whereas another fluid is used in the heat delivery loop. For instance, the heat source may be heating one type of fluid that is pumped through pipe  21  into heat exchanger pipe  102  for heat delivery to the storage  100  and via pump  65  back into the heating source for re-heating. Thermal oils may be a typical type of working fluid here. The same figure also illustrates the heat extraction mode a different type of working fluid is used for transporting heat out of the storage. Note that the heat extraction operation makes use of the same heat exchangers as for the heat storage. 
       FIG. 6  further illustrates an alternative application of the invention where the working fluid for the heat extraction, on the delivery side, is water-steam. The advantage by this is that the heat delivered can be used directly to run steam turbines and electric generators. Water-steam is heated in the pipes  103  within the heat exchangers of the storage and goes via pipe  23  into turbine  81  after which it goes through pipe  18  into a cooling system  43  and via pipe  24  and pump  62  back into the storage for re-heating. The figure indicates a cooling tower  82  as cooling system whereas other ways of cooling can also be used. There may also be other pipes, valves and pumps not shown in the figure. 
       FIG. 7  shows in principle one type of heat exchanger that may be used within the storage in connection with the application illustrated in  FIG. 6 . The working fluid  113  of the primary heating loop goes through pipe  103  which is submerged in the heat transfer fluid  112  within the heat exchanger container  120 . This container is in direct contact with the solid state heat storage material  121  consisting of one or several zones of different materials. The heat extraction makes use of fluid  114 , which is typically water-steam, via pipe  102 , into the heat utilization loop. 
       FIG. 8  shows a somewhat different and simplified version of heat exchangers used in the storage. In this case the working fluid in the heating loop  113 , fed through pipe  104 , is the same as the heat transfer fluid in the heat exchanger container. 
     It should be clear that the storage as illustrated in  FIG. 6  not only functions as heat storage; it is also a heat exchanger between two different types of working fluids where the working fluid used in the heat extraction is used directly to run electricity generating steam turbines. 
     Considering a power plant as in  FIG. 6  with two different types of working fluids it will be further understood that there is always need for a heat exchanger between the heated fluid used in the heat absorber  2  and the steam used in the turbines. In case the storage is fully bypassed without delivering heat to the storage such heat exchanger and boiler system must be provided as a separate unit on the outside of the storage (not shown in  FIG. 6 ). However, the invention provides an alternative to having an external, separate heat exchanger between the two fluids since the plant comprise heat storage with heat exchanger in itself. The objective of delivering heat generated in the primary loop directly to the turbines can in fact be achieved by running the two loops shown in  FIG. 6  simultaneously. The design of the heat exchangers illustrated in  FIGS. 7 and 8  is such that when the two fluids  113  and  114  are circulated at the same time most of the heat transfer goes directly between the two fluids rather than into the storage materials  121  via the container  120 . Some heat will necessarily be leaked from the heat transfer fluid into the storage during the targeted heat exchange operation; however, this heat leakage does not constitute any real energy loss. In fact the heat transfer to the storage may be planned as “partial storage mode” for the overall operation. The actual and relative velocities of the fluid flows decides how much heat is directly transferred and how much is stored. 
       FIGS. 9 and 10  illustrate yet another application of the invention. In this case the working fluid of heat source is directly water-steam. The figure indicates a CSP plant where water is heated to steam at very high temperature and pressure in the solar receiver. The source of heating may also be of other type such as a fossil fuel power, nuclear or geothermal power plant, replacing or in addition to the tower. In the heat storage mode shown in  FIG. 9  steam is brought from the source  2  via pipe  25 , valve  43  and pipe  26  into the pipe loop  101  of the storage. After this the fluid is pumped by way of pump  66 , pipe  27 , valve  44  and pipe  28  back into the source  2  for re-heating. 
     Prior art plants using water-steam as the only energy transport medium, can not store the high temperature heat in a practical way, since storage is impractical due to excessive number of thick walled steam pressure tanks and very high related cost. If the steam condenses, the steam is lost. With the solution of the invention, storage without practical size limits, large dynamic differential temperature ranges, higher maximum and lower minimum temperature, out of phase with source delivery, and increased maximum production level by combining delivery from storage and source, can easily be achieved. 
       FIG. 10  shows the situation for heat extraction operation. Steam is heated in pipe  101  within the heat exchangers in the storage and goes through pipe  26 , valve  43 , and pipe  29  into the electricity generating turbine  81 . After this the fluid goes via pipe  18  into cooling system  43  and is pumped via pump  62 , pipe  19 , valve  44  and pipe  28  back into the storage. 
     It is to be noted that the pipe arrangement within the storage may be arranged in different ways to serve a multiple of storage elements; the fluid flow may be organized in serial as well as parallel arrangements. The overall piping system may be adapted to running turbines directly from heating source  2  in parallel with tapping heat from the storage  100 . Although this parallel mode of operation is not shown in  FIGS. 9 and 10  it may be understood that fluid flow from pipe  25  may be combined with fluid flow from pipe  26  to provide sufficient fluid and heat for the turbines, thereby increasing maximum production. Note that the storage can have a multiple of ways of sending the working fluid through it. 
     An important consideration may be that the working fluid or heat exchanger fluid will solidify if its temperature falls below the melting point. This situation is particularly applicable to heat transfer fluids suitable for operation at very high temperatures. One way of dealing with this is to operate the system in such a way that the temperature in fluid  112  will never fall below the solidification temperature of the heat exchanger fluid. Another approach is to allow transition from liquid to solid to happen. Such transition may not necessarily imply damage to the system provided that excess thermal expansion does not take place during solidification. Unlike water, most fluids contract during solidification. In fact, phase transition may represent added heat storage capability in that transition from solid to liquid in the heating phase requires extra (stored) heat, which will be given back to the working fluid in the heat extraction mode. 
     In cases where molten salt or other fluids with relatively high melting points are used as working fluids or as heat transfer fluids in the heat exchangers in the storage problems associated with possible solidification and clogging of pipes may be dealt with in another way. As described in the above mentioned patent applications, electric heating elements may easily be built into the heat exchangers. These heating elements may be put into effect by using electricity when needed. 
     Further reference is made to  FIGS. 11-14  illustrating embodiments of the previously mentioned pipe in pipe solution, for which the at least one heat transfer container is the inner pipe and the volume between the heat transfer container, sometimes termed heat pipe, has been filled with concrete.  FIGS. 11 and 12  illustrate a basic unit or a basic heat cell, in longitudinal section and cross section, respectively. The inner and outer pipes can be seen clearly, concrete has been arranged in the volume between said pipes. The inner pipe is the heat transfer container. The means for heat input and output inside the heat transfer container can be arranged in many ways also for the so called pipe in pipe solution, as described above and illustrated for other embodiments. In some preferred embodiments the heat transfer fluid and the means for heat input and output is the same fluid.  FIGS. 13 and 14  illustrate a so called double U embodiment of the pipe in pipe solution, in longitudinal section and cross section, respectively. For clarity, the internal parts of the heat transfer containers have not been illustrated in said figures. 
     Storage utilizations beyond what has been described here will also be feasible; it is not possible to describe all possible situations. However, such applications will typically be variations on what has been described herein and may also include additions to the current invention. Also, turbines can be replaced by any kind of heat engine such as Brayton cycles, organic Rankine cycles, Kalina cycles, Stirling engine or other feasible machines for electricity production. Turbines can also be replaced with a pure heat demand, such as process heat for various industrial processes, or combinations of both. 
     The current invention may also be adapted and utilized in a hybridized power plant. Examples of this are CSP combined with natural gas, coal and biomass. Other combinations are also possible. The case of a hybrid power plant must be considered a variation of what has already been described. Accordingly, the plant of the invention can include any features or steps as here described or illustrated, in any operative combination, each such combination is an embodiment of the invention.