Abstract:
Existing approaches to refuse handling are all based on historical approaches which rely on a network of refuse collection vehicles collecting waste from individual households and delivering this to a centralised landfill or MBI location. This is highly undesirable and wasteful. An alternative process is disclosed, relying on the thermal treatment of waste and like products produced or brought in to the residential property and processed within the domestic curtilage to produce fuel or other forms of energy. Thus, domestic waste will be thermally treated at the home instead of being collected by local authorities and disposed of. The waste input put material will be loaded into a domestically engineered thermal conversion unit either directly or after a pre-process such as shredding. The feedstock will be converted into fuels by a thermal treatment, such as pyrolysis. The resultant output of oil and gas can either be stored or fed into a boiler unit to be used as a fuel to produce hot water, or used to run an electricity generating unit to power the dwelling in question or for supply to a feed-in tariff. Thus, a domestic dwelling includes a thermal treatment unit for processing waste produced in the dwelling, an output of the thermal treatment unit being combusted for producing an energy output for the dwelling. A suitable pyrolysis chamber is disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the disposal of refuse, especially refuse from domestic dwellings and smaller commercial establishments. 
       BACKGROUND ART 
       [0002]    The domestic households of the United Kingdom collectively generate approximately 30 million tonnes of waste per year. Typically, 40% of this is recyclable—such as paper, cardboard, glass, cans plastics and (in some cases) green waste and food waste. This waste is collected from the majority of households via a kerbside type collection scheme operated by or on behalf of local government bodies. Existing UK practice is to require householders to divide waste into separate containers for the various types of recyclable waste, to allow these to be harvested with the remainder being classed as residual waste. The waste is collected either weekly or fortnightly, and the residual waste is either landfilled or sent to large scale Energy from Waste plants to generate electricity. 
         [0003]    The current approach is that waste is collected in Refuse Collection Vehicles (RCV) and disposed of through large-scale Energy from Waste (EfW) incinerators or Mass Burn Incinerators (MBI). These can process circa 200k to 300k tonnes of waste per annum. These MBIs use existing proven technology which makes them attractive to the financial markets to fund the capital cost of the plant when combined with long term waste disposal contracts with the County Councils who are responsible for the disposal and treatment of waste generated by their residents. 
         [0004]    On the fringes there are alternative thermal treatment technologies emerging, such as gasification, pyrolysis and plasma, which effectively break down the waste to a synthesised gas (Syngas) and fuel product. The gas and oil can then be utilised to run electrical generating equipment. These technologies are still in their relative infancy but do have substantial environmental benefits; they are cleaner than the conventional incinerators and although smaller than MBIs their capacity is still substantial, circa 15k to 30k tonnes per annum. However, it is not yet clear that these units will be able to scale their capacity up to a commercially viable size. 
         [0005]    US 2007/0099039A1 discloses an appliance for converting household waste into energy, in which syngas is used to power a fuel cell for the generation of electric power, steam and heat or cooling for use in residences and buildings. A waste conversion reactor that applies a steam reforming process to the waste is heated with a combination of waste heat and electrical power or a natural gas burner. To our knowledge, this steam reforming process has not been shown to be practically usable, especially in a domestic context. 
       SUMMARY OF THE INVENTION 
       [0006]    The currently-viable approaches to refuse handling are all based on historical approaches which rely on a network of RCVs collecting waste from individual households and delivering this to a centralised landfill or MBI location. This is highly undesirable and wasteful. A substantial investment needs to be made in a fleet of RCVs, which also incurs significant running costs. The process of operating these vehicles introduces a range of health &amp; safety issues; injuries can result from the lifting, moving and handling work that is required, and road traffic incidents arise through placing operatives in the roadway around both routine traffic and the RCV itself. A location needs to be found for the landfill or MBI, which needs to comply with local planning or zoning laws and which usually meets with resistance from nearby residents. To obtain an acceptable site for the landfill or MBI, a remote location with few immediate neighbours is usually needed, thereby increasing the running costs and associated emissions of the RCV fleet. These historic approaches also assume that there is a ready market for recyclable materials, which has previously been provided by demand from Asian countries. However, as those countries become more self-sufficient in collecting and processing their own recyclable waste, this market is declining. 
         [0007]    The present invention therefore proposes the thermal treatment of waste and like products produced or brought in to the residential property and processed within the domestic curtilage to produce fuel or other forms of energy. Thus, domestic waste will be thermally treated at the home instead of being collected by local authorities and disposed of. 
         [0008]    The waste input material will be loaded into a domestically engineered thermal conversion unit either directly or after a pre-process such as shredding. The feedstock will be converted into fuels by a thermal treatment, such as pyrolysis. The resultant output of oil and gas can either be stored, or fed into a boiler unit (or combusted in situ) to be used as a fuel to produce hot water, or used to run an electricity generating unit to power the dwelling in question. It can also be used as a fuel to assist in obtaining the temperatures needed for the pyrolysis process, thereby reducing the energy demands of the unit. Some or all of the electricity generated, or of the oil and/or gas output, could be exported for supply to a feed-in tariff, for use elsewhere. 
         [0009]    In one aspect, the present invention therefore proposes a domestic dwelling including a thermal treatment unit for processing waste produced in the dwelling, an output of the thermal treatment unit being combusted for producing an energy output for the dwelling. The output of the thermal treatment unit can include syngas (synthesis gas) and/or oil; this can be combusted in situ or fed to a separate combustor. One advantage of a separate combustor is that it can be adapted to receive a second fuel in addition to the combustible output, for example in order to allow continuity of fuel supply regardless of the amount of waste being processed. The combustor will often be a furnace (or boiler as referred to in the UK) connected to pipework installed within the dwelling for heating water circulating within the pipework. That water can be used to heat radiators and/or a hot water supply for the dwelling. Alternatively, the combustor may be an electrical generator or a combined heat and power unit. 
         [0010]    The output of the thermal treatment unit often includes a residue, so the thermal treatment unit can have a connection to a sewerage system to allow disposal thereof. 
         [0011]    In another aspect, the present invention provides a pyrolysis chamber for treating domestic refuse, comprising a receptacle for the refuse, the receptacle being defined by a double-skinned enclosure having an exterior wall, an interior heat-conductive wall, and a void between the two walls in which a working fluid is disposed, and at least one heating element in thermal contact with the enclosure. 
         [0012]    The double-skinned arrangement with a working fluid within the void creates a chamber with a very uniform temperature distribution, which assists the pyrolysis process. 
         [0013]    The enclosure preferably also includes at least one heat-conductive pin extending from the interior wall into the interior of the receptacle, to create a more uniform temperature distribution within the interior of the chamber and especially within the interior of the initially-cold refuse placed within the chamber. The at least one pin is preferably hollow, and the hollow interior of the pin is ideally in fluid communication with the void in order to share the heat-transmitting capabilities of the fluid. 
         [0014]    The receptacle preferably also includes at least one leg extending from the exterior wail, and a heat source connected to the leg in order to introduce heat into the chamber and elevate the temperature to a level sufficient for pyrolysis. The at least one leg is ideally hollow, with the hollow interior of the leg being being in fluid communication with the void in order to provide a high degree of thermal communication with the remainder of the chamber. The heat source can be an electrical heating element, or a heat exchanger, which may be supplied with waste heat recovered from the current or previous pyrolysis cycles. Multiple such legs may be provided, each with a heat source. A range of different heat sources may be provided on different legs. 
         [0015]    The enclosure is preferably formed of stainless steel in order to provide the necessary thermal, mechanical and corrosion properties. 
         [0016]    The working fluid need not completely fill the void; an expansion gap may be provided which can be evacuated, or filled with an inert gas, or filled with air. The working fluid can be Dowtherm A™ or (liquid) sodium; generally Dowtherm A™ is preferred for temperatures up to about 400° C. whereas sodium is preferable above this level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    An embodiment of the present invention will now be described by way of example, with reference to the accompanying figures in which; 
           [0018]      FIG. 1  shows a typical domestic dwelling in which the present invention has been installed; 
           [0019]      FIG. 2  shows a system schematic according to the present invention; 
           [0020]      FIG. 3  shows a side view of a pyrolysis chamber according to the present invention; 
           [0021]      FIG. 4  shows a vertical section through  FIG. 3 , on line IV-IV; 
           [0022]      FIG. 5  shows a horizontal section through  FIG. 3 , on line V-V; 
           [0023]      FIG. 6  shows a horizontal section through  FIG. 3 , on line VI-VI; 
           [0024]      FIG. 7  shows an enlarged version of region VII on  FIG. 4 ; and 
           [0025]      FIG. 8  is a process flowchart. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0026]    The current systems of domestic refuse collection and disposal still rely on one common thread—that waste needs to be collected from every household. Collecting waste is expensive, not only in terms of the immediate costs of collection, disposal and/or processing, but also in terms of national or regional environmental taxes relating to dispose of waste; in the United Kingdom this amounts to approximately £90 per tonne. The industry trend is in making improvements in the handling of the waste after it has been collected and taken to a processing site—improvements such as better sorting of the waste to increase recycling rates, and/or alternative disposal methods such as mass-burn incinerators (MBIs) and thermal treatment methods including pyrolysis and plasma treatment. However, MBIs are expensive, and the emerging thermal technologies have not been demonstrated to be capable of being scaled up to an economically viable capacity. 
         [0027]    According to the present invention, such thermal technologies are scaled down, instead of up. Instead of a large unit at a processing site to which the refuse must be transported, the invention provides a domestic unit, located at the house, to process the waste produced in that house. Waste is put into a domestic thermal treatment unit, ideally about the size of a white-goods appliance, which would process the waste and generate waste oil and syngas. The oil and syngas could then fuel a domestic boiler, probably supplemented by a natural gas supply in order to ensure a stable supply. Alternatively, or in addition, the oil and gas could be used to run a small-scale electricity generating unit to produce electrical power. This could then be stored via batteries and used when required, or fed into the household power mains, or fed into the local electrical grid via a feed-in tariff. So-called “micro combined heat and power” are available for domestic use and produce a combination of heat for the dwelling and an electrical power output. The combination of energy generated from waste and solar technology to generate electricity during the day, along with new efficient building design, could make houses self-sufficient in time. The thermal treatment unit will also provide a direct heat output which can be piped throughout the house as a process by-product, or integrated into the heating capability of the boiler, or used to drive an electrical generation unit directly. 
         [0028]    A small-scale unit of this type could use one or more of plasma, pyrolysis and cracking type technologies, although we believe pyrolysis to be the most practical. These take a mixed homogeneous material stream and use thermal and pressure based processes to reduce the waste composition back to 3 component parts—oil, gas and carbon. Pyrolysis technologies that are currently available use heat in the absence of air to decompose the molecular composition of the material. Plasma gasification is a process which converts organic matter into synthetic gas, electricity, and a solid residue using plasma. A plasma torch powered by an electric arc is used to ionize gas and catalyse the conversion of organic matter into synthetic gas and solid waste, 
         [0029]    The waste can be shredded in order to make it more suitable for processing in the unit. A receptacle for the waste, either before or after a shredder, will help smooth the flow of waste into the unit. 
         [0030]      FIG. 1  shows a domestic dwelling-house  10  in which the system is installed. A roadway  12  has a pavement (or sidewalk)  14  over which access can be gained to a driveway  16  in front of the house  10 . A walkway  18  alongside the driveway  16  allows access to a front door at the front of the house  10 . A perimeter fence  20  extends along one side  22  of the property  24  in which the house  10  is located, then along the rear boundary  26 , and along the opposite side boundary  28  as far as the pavement  14 . Together with the pavement  14 , the perimeter fence  20  provides a visible definition to the boundary of the property  24 . 
         [0031]    The house  10  itself has an external wall  30  and internal walls  32  defining individual rooms within the house, including a hallway  34  into which the front door opens, a sitting room  36 , a dining room  38 , a kitchen  40 , and a garage  42  which opens onto the driveway  16 . Stairs  44  lead upwards from the hallway  34  to an upper floor with bedrooms and one or more bathrooms. The house may of course be arranged differently and have more or fewer rooms. 
         [0032]    A thermal treatment unit  46  is located adjacent the house  10 , against the external wall  30 . It is located near to the kitchen  40  where most of the waste is produced, for the convenience of the householder. As noted above, the thermal treatment unit  46  produces an output that is a combination of syngas and combustible oils, these are fed to a boiler  48  which (in this case) is in the conventional location within the garage  42 . The boiler supplements the output of the thermal treatment unit  46  when necessary with a natural gas supply, and burns these fuels in order to generate heat for the house  10  in a conventional manner. This is shown in  FIG. 2 ; waste  50  is placed in a receptacle  52  in the thermal treatment unit  46 , and passes through a shredder  54  before being treated. The output of the treatment unit  46  is then fed to the boiler  48  and is used to heat a working fluid (usually water) that is circulated around one or more heating circuits. The circuit may include a hot water tank  56  containing water that is then heated for use within the house  10 , and/or a number of radiators  58  for warming the house  10 . 
         [0033]    The boiler  48  may be in the form of a micro-combined-heat-and-power unit, and thus may also have an electrical output  60  leading (eventually) to one or more electrical outlets  62 ,  64  within the house. The or another electrical output  66  may also lead to a connector for a feed-in tariff, in which locally generated power is fed into the wider electrical grid. 
         [0034]    Of course, the thermal treatment unit  46  and the boiler  48  may be integrated into a single unit that accepts waste  50  and (in all likelihood) an alternative power source (such as a natural gas supply), and creates heat and/or power for the house  10 . Likewise, the unit or units may be located differently within (or outside) the house. 
         [0035]    Pyrolysis units typically leave a residue of carbonised material, usually as a light ash. This is inert, and can thus be flushed away via a sewerage system. The thermal treatment unit  46  therefore has (in preferred arrangements) a connection to the household water supply and sewerage system for this purpose. An alternative is to drop the residue into a container for routine disposal by the householder. 
         [0036]    This gives rise to a wide range of benefits. Fewer waste collections are needed, resulting in savings in the waste infrastructure and landfill usage, and reductions in the greenhouse gas emissions from landfill sites. The emissions from refuse collection vehicles will also be reduced, as will the fossil fuels used by them, leading to reduced operating costs for local authorities and the potential for reductions in local taxes. At the same time, once the initial capital costs have been covered, energy bills for the households in question will be reduced corresponding to the heat and/or electrical power produced by the unit. Fossil fuel usage by households and by the power generation facilities that supply them will also reduce. 
         [0037]      FIGS. 3 to 7  illustrate a suitable pyrolysis chamber for use in the present invention. Thus, a generally cylindrical vessel  100  has circular side walls  102  and an integral base section  104 . An open upper end is closeable during use by way of a lid  106 , held in place via a hinge  108  on one side and a latch mechanism  110  opposite, to allow the lid to seal hermetically to the vessel  100 . An inlet  112  is formed in the lid  106  to allow water for cooling and washing to be introduced, and a corresponding outlet  114  is provided on the base  104  of the vessel  100 . If preferred, the inlet  112  could be provided elsewhere, such as on a side wall of the vessel  100 . An exhaust port  116  is formed on the side of the vessel  100  to allow for extraction of the pyrolysis results, as will be explained below. 
         [0038]    Within the vessel  100 , there are five spikes  118  extending upwardly from the base  104 , arranged in a square formation with four spikes at the corners of the square and one at the centre, the square being centred on the central axis of the cylindrical vessel. The diagonal of the square is approximately half the diameter of the cylindrical vessel  100 , thus distributing the spikes  118  evenly around the interior of the vessel  100 . The exhaust port  116  faces a side of the square, thereby minimising the effect of the spikes  118  on flow through the exhaust port  116 . These spikes  118  fit between and into refuse that has been loaded into the vessel  100  and serve to introduce heat from the vessel surfaces into the bulk of the refuse, thus assisting with the pyrolysis process. More or fewer spikes could be provided as desired, with a greater number of spikes providing more effective heat transfer and a lesser number making loading of refuse into the vessel  100  easier. A narrower vessel  100  may require fewer spikes  118  in order to ensure temperatures adequate for pyrolysis at the middle of the vessel; indeed, it may be possible to omit the spikes  118  entirely. Likewise, the spikes  118  could be arranged differently, although symmetrical arrangements are to be preferred. 
         [0039]    Four legs  120  extend from the exterior of the base  104 ; these are again arranged in a square pattern centred on the central axis of the cylindrical vessel, with each leg at a corner of the square and the diagonal of the square being approximately half the diameter of the cylindrical vessel  100 . The square pattern of the legs  120  is however rotated 45° relative to the square pattern of the spikes  118 ; as a result the legs  120  and the spikes  118  are not directly aligned. Each leg has an associated heat source; for three legs the heat source is an electrical heater  122  with contacts  124 ,  126  through which electrical power is supplied in order to heat the leg. The fourth leg is provided with a heat exchanger  128  which will be described further below. 
         [0040]    The walls of the vessel  100  are formed in a double-skinned arrangement, including a thermally-conductive inner wall  130  and an outer wall  132 . The outer wall  132  is ideally also thermally conductive; indeed as illustrated in  FIGS. 3 to 7  it is an integral structure with the inner wall  130 . A high-grade stainless steel is suitable for the wall material as it is sufficiently rigid and thermally conductive, while offering resistance to corrosion due to the potentially harsh environment that may be created within the chamber  100  during pyrolysis. The outlet  114  and the exhaust port  116  extend from the inner wall  130 , through the void  134  between the two walls, and through the outer wall  132 . The spikes  118  and the legs  120  extend from the inner wall  130  or the outer wall  132 , respectively, and are themselves hollow with their hollow interior  136  communicating with the void  134 . 
         [0041]    Within the void  134 , and thus also within the hollow interiors  136  of the spikes  118  and the legs  120 , there is a small amount of heat transfer fluid (not shown). 
         [0042]    The walls of the waste treatment chamber are thus formed by a hermetically-sealed, passive two phase heat transfer system, which is partially filled with a small amount of working fluid, present inside the chamber in two phases (liquid and vapour). The working fluid is substantially the only substance in the chamber&#39;s enclosure and is ideally saturated under all thermal conditions of the chamber. The superior thermal characteristics of the chamber make use of the highly efficient heat transfer processes of evaporation and condensation to maximize the thermal energy transfer between the heat legs  120  at the bottom and inner walls  130  of the chamber  100  that are in direct contact with the waste to be treated. The heat applied on the four legs of the chamber section is conducted across the leg&#39;s walls, causing the working fluid in the enclosure to boil. In this way the working fluid absorbs the applied heat load, storing it at least partly as latent heat. The vapour then flows to the (cooler) walls above the level of the legs  120 , where it can condense and release the latent heat. The heat then conducts through the thin liquid film and the metal wall  130  and into the waste treatment chamber. Within the void  134 , the flow circuit is completed by the liquid being forced by gravity back to the legs  120  in the form of a thin liquid film where it can be re-evaporated. The working fluid within the enclosure is at a substantially constant temperature (called the working temperature); thus when the vapour condenses on the inside walls of the chamber, it will keep these walls at the same temperature leading to the surfaces that will be in touch with the waste all being in a substantially single isothermal state. 
         [0043]    The required working fluid volume will vary based on the thermal requirement, but is likely to be between 10% and 35% of the chamber&#39;s enclosure volume (i.e. the void  134  between the walls  130 ,  132  that form the chamber  100 ). Based on the required wall temperature of the chamber, the selection of the working fluid will be as follows: 
         [0044]    a. Sodium or chamber wall temperatures between 400° C. and 700° C. 
         [0045]    b. Dowtherm A™ for chamber wall temperatures between 300° C. and 400° C. 
         [0046]    c. Water for chamber wall temperatures below 300° C. 
         [0047]    The Dowtherm A heat transfer fluid is described by its manufacturer at http://www.dow.com/heattrans/products/synthetic/dowtherm.htm, and is a eutectic mixture of biphenyl (C 12 H 10 ) and diphenyl oxide (C 12 H 10 O). The fluid also includes a dye to give it a clear to light yellow colour, but this is purely to aid in leak detection and although useful, is not essential to the invention. Dowtherm A fluid is described as being useful in systems employing either liquid phase or vapor phase heating, including indirect heat transfer. Generally, Dowtherm A will be sufficient for the temperatures usually required for pyrolysis processes, but if higher temperatures are required then liquid sodium can be used. Dowtherm A is to be preferred, however, as it does not present an explosion risk if the double walls of the chamber are breached. 
         [0048]    The double-walled nature of the vessel  100  and the heat transfer fluid within the void  134  ensure that an even and high temperature can be achieved across the vessel  100 . The spikes  118  extend that even temperature into the interior of the vessel  100  and accelerate the pyrolysis process significantly. The legs  120 , with their hollow interiors that can accept the heat transfer fluid, are an especially efficient way to convey heat to the remainder of the vessel  100 . 
         [0049]    The chamber will ideally be located within a suitable enclosure, which will also house the various supporting ancillaries such as control systems for the heating elements, etc. 
         [0050]      FIG. 8  shows a system diagram of the refuse treatment unit. The pyrolysis vessel  100  is opened via the lid  106  and pyrolisable refuse is placed inside. The lid  106  is closed and latched; oxygen is then removed from the vessel by either evacuation or displacement with an inert gas. The electrical heaters  122  are activated and the temperature within the vessel  100  is raised to a suitable temperature for pyrolysis, usually above about 170° C. As pyrolysis progresses, the chamber will yield water vapour and syngas via the exhaust port  116 . This is passed to a multi-stage condensing and heat recovery unit  200 , which essentially acts as a heat exchanger to heat water and/or air delivered via an inlet  202  and expelled via an outlet  204  to provide heated water and/or air for heating and/or sanitary use within the home or other building in which the unit is installed. As a result, the significant power requirements of the endothermic pyrolysis process is not wasted, but is re-used for other purposes thereby reducing the overall energy footprint of the device. 
         [0051]    After heat recovery, the syngas and water mixture is further cooled, and thus separated by condensing of the water vapour, in a cooler  300 . The syngas output of this cooler  300  is diverted via a valve  302  either to a storage tank  304  or to a combustion chamber  306  where the syngas can be burnt in a feed of air  308 . Heat from the combusting syngas can then be supplied (using a suitable working fluid) to either the heat exchanger  128  on one or more legs  120  of the pyrolysis vessel  100  or to a waste heat recovery unit  400  via a valve  308 . The waste heat recovery unit  400  also heats a domestic water or air supply  402  to produce a supply of hot water and/or air  404  for use in the same way as the hot water and/or air supply  204  above. The heat exchanger  128  and the waste heat recovery unit both vent any excess heat to a radiator  500  that releases the heat to the ambient environment; in addition, if necessary the heat output of the combustion chamber can be vented in the same way via a valve  310  prior to the waste heat recovery unti  400 . 
         [0052]    Thus, the pyrolysis chamber  100  requires a supply of refuse and heat; it outputs syngas, water vapour and heat. The output heat is recovered for domestic purposes, meaning that it may be preferable to store refuse in or near the pyrolysis unit until heating and/or hot water is needed. The syngas is extracted from the cooled output stream and can be stored until needed, or used immediately; once called upon it can be used to heat the building in question, or to contribute to the heat input called for by the vessel  100 . In the latter case, the syngas can be stored for use in a later pyrolysis process, or may be used immediately in support of a later part of the current pyrolysis process. 
         [0053]    Thus, the unit can be operated in a number of modes. Given that energy will be required to heat the building, that energy can be diverted to the pyrolysis process and then recovered to provide heating; together with the contribution from combusting syngas, this means that the net overall energy requirement of the unit could be very small. Alternatively, the unit could be driven substantially entirely via electrical heating, from which heat can be recovered in order to heat the building and syngas could be generated to further heat the building. Either way, the domestic refuse is disposed of, reducing or eliminating the need for a doorstep refuse collection (with the comcomitant environment benefits set out above), for little energy cost and/or with a benefit in the form of a heat supply for the building. 
         [0054]    Once pyrolysis is complete, water can be injected into the vessel via the inlet  112  to cool the vessel  100  to a safe temperature and flush out solid residues via the outlet  114 . These residues may be usable as biofuel, depending on the nature of the refuse that was initially put into the unit. Careful control of the working temperature of the waste treatment chamber should be sufficient to optimise the biofuel output. 
         [0055]    It will of course be understood that many variations may be made to the above-described embodiment without departing from the scope of the present invention. For example, the thermal treatment unit, the shredder and/or the combustor could be adapted to utilise bio fuels. The combustor could be adapted to utilise natural gas or oil, and/or the electricity generating unit could be adapted to supplement its output derived from the combustor with electricity derived from wind, solar or geothermal energy sources. The domestic radiators referred to above could be any system which uses heated water to raise the temperature in a dwelling, such as underfloor heating. Other than heating the dwelling directly, the system could provide indirect heating, such as by using the electricity generated to operate a geothermal heating system via a heat pump. Furthermore, where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.