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Micro combined heat and power (CHP) systems for residential and small commercial buildings
13
Micro combined heat and power (CHP) systems for residential and small commercial buildings
J. H a r r i s o n, E.ON Engineering, UK
Abstract: The principal market for micro-CHP is as a replacement for gas boilers in the 18 million or so existing homes in the UK currently provided with gas-fired central heating systems. In addition there are a significant number of potential applications of micro-CHP in small commercial and residential buildings. In order to gain the optimum benefit from micro-CHP, it is essential to ensure that an appropriate technology is selected to integrate with the energy systems of the building. This chapter describes the key characteristics of the leading micro-CHP technologies, external and internal combustion engines and fuel cells, and how these align with the relevant applications. Key words: Stirling engine, internal combustion engine, fuel cells, microCHP.
13.1
Introduction
It is widely held that the principal market for micro-CHP is as a direct replacement of around 12 million of the 18 million gas central heating boilers, representing potential annual sales of up to 1.5 million units. This is based on the premise of simple economic payback of the investment cost of the product recoverable within an acceptable period for a 1 kWe Stirling engine micro-CHP package.1 Whilst a useful metric, it is rather simplistic, does not hold true for other micro-CHP technologies, and takes no account of potential applications in other building types. Micro-CHP derives its principal environmental and economic benefit from generating electricity as a byproduct of an existing thermal load. It replaces the gas boiler in a hydronic central heating system, producing space and water heating just as the boiler might do. It requires a primary fuel input and does not claim to be renewable, but is a low carbon and extremely energy efficient technology. As might be expected, the micro-CHP product has a higher initial cost than the boiler it replaces and must recover this cost from the value of the electricity it produces. Clearly, the more electricity that can be produced from a given thermal load, the higher the electrical output and the consequent operational income. However, it is almost invariably true 325 © Woodhead Publishing Limited, 2011
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that the higher the electrical output, the higher the capital cost so that here is both a compromise in the quest for electrical efficiency in producing a product at an appropriate cost and a natural match for different technologies to different thermal loads. The principal technologies considered relevant to mass market micro-CHP are internal combustion engines (ICE), external combustion engines (such as Stirling engines) and fuel cells. Of the latter, the high electrical efficiency and potential for low capital cost make SOFC (solid oxide fuel cells) the most promising fuel cell technology, although these are still at a relatively early stage of development and there are no commercially available products in the UK or Europe. Each of these technologies has characteristics making it more or less applicable to differing building types and thermal load profiles. However, micro-CHP is still a relatively immature technology and products continue to be developed to fulfil the requirements of various market sectors, able to compete more effectively with alternatives such as gas boilers, heat pump and larger-scale CHP technologies.
13.2
Basic issues and energy requirements
Although there are certain characteristics which make applications more or less attractive and viable, there are also some fundamental criteria which must be fulfilled to make an application viable. These include fuel availability, adequate thermal demands and an accommodating regulatory environment.
13.2.1 Economic rationale for micro-CHP The most fundamental economic factor required for viability of micro-CHP is that it must be able to recover the initial investment from the value of the electricity produced in lieu of heat from the primary fuel. It therefore holds true that the value of electricity must exceed that of heat by a significant amount. The ratio of electricity and gas prices is commonly known as ‘spark spread’ and, whilst it is generally the case that this ratio is around 3:1, reflecting in part the dominance of gas as a primary fuel input to central generating plant, it is not the case in countries such as Norway and Sweden where there is no widely available natural gas network, but an abundance of low cost hydroelectric power. In countries such as the UK, however, where retail electricity prices are around 11p/kWh and gas 3.5p/kWh, the cost of heat produced in a gas boiler with 90% efficiency is just under 4p/kWh. Thus, assuming the total efficiency of the micro-CHP unit to be equivalent to that of the gas boiler, each unit of electricity produced costs the consumer the lost opportunity cost of one unit of heat (4p), but is worth the value of displaced electricity
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(11p) assuming further that the electricity is consumed within the home and not exported. Each unit of electricity thus generates a net income of 7p; an annual production of 3000 kWh is worth up to £210. For domestic investments, this could justify a marginal capital cost of around £600 for the mass market of consumers who require a three-year payback and up to £1000 for a smaller proportion of the market who are satisfied with a payback of five years. Considering that the average UK householder moves home every seven to eight years on average, even an investment of £1500 would seem rational and early adopters of other microgeneration technologies such as PV have been prepared to invest with paybacks in excess of a century as an environmental gesture! Figure 13.1 illustrates the simplified concept of electricity generation at the expense of heat for the major micro-CHP technologies. The importance of this is that the value of each unit of electricity produced is the same (in both economic and environmental terms) regardless of the technology or its electrical efficiency; a higher electrical efficiency product will generate more units of electricity for a given heat load (and will consequently consume proportionately more gas) so that its overall income is higher, but the value per kWh of electricity is exactly the same. This belies the widely held, but erroneous view that low efficiency micro-CHP technologies are somehow environmentally inferior to higher electrical efficiency products. Indeed, the reality for currently available, high electrical efficiency ICE products is that they do not achieve the same total efficiency as gas boilers so that the saving per unit electricity generated is reduced by the cost of additional gas burned. It is of course necessary that for economic viability, the micro-CHP unit must run for enough hours each year to generate sufficient electricity to
Solid oxide fuel cell
IC engine
Heat Electricity Loss
Stirling engine
Gas boiler 0%
20%
40%
60%
80%
100%
13.1 Economic rationale for micro-CHP.
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recover the initial investment. That requires an extended heating period; not necessarily a high peak demand, but preferably a long moderate heat demand in order to make full use of the heat demand to generate electricity for at least 2500 hours annually. However, technologies currently under development such as fuel cells with very low heat-to-power ratios may be able to achieve economic viability for homes with much smaller thermal demands so that some SOFC technologies are able to operate continuous baseload to meet domestic hot water (DHW) demand with an electrical output of 1 kWe every hour of the year.
13.2.2 Fuel availability The majority of micro-CHP products are designed to operate using natural gas as a fuel input and this represents the substantial market in Europe. Although it is possible to operate micro-CHP using alternative fuels such as liquid petroleum gas (LPG), fuel oil and even biofuels,2 these are relatively expensive and do not constitute a significant potential market. It is therefore natural that access to a natural gas network is an essential requirement for mass market micro-CHP applications. In the UK the majority (18 million of the 24 million or so homes) are indeed connected to the natural gas network and are equipped with gas-fired central heating. Germany and the Netherlands also have a majority of heating systems based on hydronic natural gas central heating.
13.2.3 Regulatory environment Although not essential, it is certainly preferable that the energy market is liberalised, allowing connection of micro-CHP units to the LV electricity network and being able to provide rewards for the export of electricity to the grid. In Japan, where it is not possible to recover value for any excess generation exported to the grid, products are designed to deliver sub-optimal performance to avoid export. Although such products are made viable by the application of grants and other subsidies, the liberalised markets of many European states offer a much better prospect of sustainable economic viability.
13.2.4 Technical requirements for micro-CHP viability In addition to these criteria for economic viability, there are also physical constraints on the technologies themselves as they must be suitable for installation either in the home or in other appropriate buildings. Micro-CHP is intrinsically different from conventional CHP in that it serves the highly volatile thermal and electrical loads in individual homes
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in contrast to the more stable demands arising from the diversified loads characteristic of larger systems serving multiple homes or other building types. Therefore it is implicit that power cannot be wholly provided from CHP and that such systems will, in the majority of circumstances, be grid connected and exchange electrical energy with the network. Residential electrical loads fluctuate from below 100 We and can peak at over 20 kWe. Another complication is that residential heating systems typically operate for only around 2500 hours per year, much less than the normal CHP criteria (typically 6000 hours) for commercial viability. In addition, most manufacturers recognise it is not acceptable either practically or economically to service units more often than once annually, such as is required for gas boilers. Taken together these constraints impose severe technical demands on micro-CHP which are only just being resolved. For commercial buildings, nursing homes and similar buildings it is normal to have space allocated to a plant room which may contain bulky, relatively noisy equipment with regular access for service; in such applications ICE technology may be most appropriate particularly as this technology is able to deliver relatively high electrical efficiencies from a wide range of manufacturers. For individual homes, however, it is necessary to incorporate the micro-CHP product within the occupied space so that noise, vibration and physical bulk must be limited, as must intrusive service access requirements. It is therefore not possible to consider all technologies as equally suitable for all applications, although as a rule, technologies suitable for domestic installation are likely to be suitable also for commercial plant room applications subject to the necessary cost criteria.
13.3
Types of system for residential and small commercial buildings
Although all micro-CHP products have the common characteristic of producing heat and power from a primary fuel, fossil or otherwise, there are many different technologies acting as the ‘prime mover’, each with its own particular characteristics which make it more or less suitable for any given application. Alternatives, discussed in greater detail elsewhere in this book, include various types of engines, fuel cells, turbines and novel devices such as thermo-electric convertors.
13.3.1 External combustion engines External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work. The continuous, controlled external combustion process offers significant advantages in terms of low
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emissions, high efficiency, low noise and vibration and potentially long life and extended service intervals, although it must be conceded that these characteristics have yet to be demonstrated in the relatively immature products currently approaching commercialisation. Stirling engines have long been considered the leading micro-CHP technology, but have so far failed to reach the market in significant numbers, although it is expected that both WhisperGen and Baxi will launch their mass-produced 1 kWe products on the UK market during 2010 through the energy companies E.ON and British Gas, respectively. Both products have an electrical output around 1 kWe and electrical efficiencies approaching 15% and with overall efficiencies similar to those of gas boilers. For a typical UK home they would be expected to generate around 3000 kWh of low carbon electricity with a value of up to £200 if all the generated power were consumed on site and an additional £300 from the proposed FIT (feed-in tariff) subsidy. However, it should be noted that promoters of these technologies recommend their installation in larger homes with higher than average thermal demands in order to maximise the income from electricity generation and minimise payback; such homes, with an annual thermal demand of greater than 18 000 kWh constitute around half of the gas centrally heated homes in the UK.
13.3.2 Internal combustion engines Internal combustion engines inject fuel and air into the cylinders where combustion occurs. The resulting temperature and pressure changes of the fuel/air mixture (which is also the working gas) act on the piston to produce useful work. This is mature technology, able to draw on extensive experience in both stationary and automotive applications, although the characteristic high emissions, noise and vibration as well as high service requirements inherent in this technology, raise significant challenges for micro-CHP applications. However, current products available from as little as 1 kWe and with a range of outputs up to 50 kWe (the upper limit of microgeneration*) seem to have substantially overcome each of these challenges and there are around 100 000 Honda Ecowill (1 kWe) units installed in Japanese homes and over 10 000 Baxi Dachs (5 kWe) units installed in large homes and similar buildings in Europe. With an electrical efficiency in excess of 25%, the electrical output of these products in many homes can be quite substantial, possibly as much as 4000 kWh per year and generating an income of around £240† (excluding *As defined by the UK Government in the Climate Change and Sustainable Energy Act 2007. † As the total efficiency of the leading current ICE product is significantly less than that of a gas boiler, the higher production cost of each unit of electricity results in a net income of only 6p.
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subsidies which could add another £400 in the UK). The relatively low heat to power ratio of 3:1 requires the inclusion of a supplementary burner in the micro-CHP package, but makes installation in lower thermal demand homes economically viable.
13.3.3 Fuel cells In a fuel cell, the chemical energy within the fuel is converted directly into electricity (with byproducts of heat and water) without any mechanical drive or generator. Generally speaking they have a much higher electrical conversion efficiency than other technologies, particularly in the case of SOFC technologies, but are relatively inflexible in performance, requiring more or less continuous operation to avoid thermal cycling and the consequent induced mechanical stresses. The leading SOFC technology with an electrical efficiency of 60% and a heat to power ratio of 1:2 operates to provide DHW (domestic hot water) throughout the year, any requirement for space heating being met by the supplementary burner included within the package. In theory, this product would be technically suitable for every gas heated home in the country, although it is a relatively bulky product, requiring a substantial thermal store to capture the ‘waste’ thermal energy in the form of hot water and is thus probably best suited to homes with the necessary available space.
13.3.4 Other novel technologies There are numerous experimental technologies which may at some future date result in useable products. These include thermo-ionic and thermoelectric technologies which utilise temperature difference acting on metals or semi-conductors to produce electricity together with thermo-photovoltaic units which convert the radiant energy emitted by the burner to produce electricity using infra-red sensitive PV cells.
13.4
Domestic applications for micro combined heat and power (CHP)
There is currently a limited number of prime mover technologies on which micro-CHP systems suitable for individual homes can be based. Given the relative technical immaturity of micro-CHP, it is not always currently possible to identify a micro-CHP product ideally matched to the specific application. The mass market for micro-CHP is for stationary, on-grid applications, primarily in domestic buildings ranging from individual detached houses to multi-storey, apartment blocks. It should be acknowledged that, for some of these applications, particularly very high density apartments, some form
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of communal CHP system may be more appropriate from an economic and environmental perspective, although even in such instances, the desire for independence and personal control may make micro-CHP the preferred option from the user’s point of view. As noted earlier, by far the largest potential market for micro-CHP is for installation in individual homes which currently have gas-fuelled hydronic central heating systems. The technology is particularly relevant to provide heating in larger, less energy efficient homes where all practical, cost effective measures have already been taken to reduce energy cost, but where a significant thermal demand remains, so-called ‘hard to treat’ homes.
13.4.1 Complementary applications of micro-CHP in housing Currently available micro-CHP products simply provide space and water heating together with electricity. This limits their operating hours to periods when space heating is required in winter plus an additional limited duration outside the heating season, perhaps an hour or so daily, when there is still a demand for hot water. Some commentators have suggested measures to enhance the performance of their micro-CHP concepts by extending the potential operating hours to meet other heat-based loads. In theory, opportunities do exist for more complex, hybrid packages including cooling to make use of the heat output and thus generate more electricity outside the heating season. However, in practice, absorption cooling has yet to be demonstrated successfully at this scale. Still others propose the application of the generated electricity to power a vapour compression heat pump to maximise the thermal output of the unit, either on a continuous basis or for peaking purposes; this same principle could be applied as a fuel arbitrage measure in the longer term when the dominant domestic heating technology may be air source heat pumps. During periods of limited electricity availability from intermittent renewable sources, effectively the micro-CHP unit becomes a component of a bivalent heat pump heating package.
13.4.2 The existing homes market It has already been explained that the micro-CHP technologies currently under consideration are assumed to replace the boiler in a hydronic system. The majority of UK homes (18 million out of 25 million) are equipped with such central heating systems, as are those of the Netherlands and Germany, generally assumed to be the other two key European markets. At the same time the majority of the 1.5 million gas boilers sold each year are for replacement of boilers in existing systems when they reach the end of their useful life, typically after 10–15 years. It therefore seems logical to focus
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marketing activities on this substantial market provided that the micro-CHP technology matches the thermal characteristics of the home. Not only do existing homes tend to have relatively higher thermal demands than newer, better insulated homes, but the heat-to-power ratio also aligns well with that of Stirling-based systems currently being introduced to the market. Around 85% of energy use within the home is for space and water heating, the remaining 15% is electricity3, as shown in Fig. 13.2. Thus the output of the micro-CHP unit, matching as it does this ratio, has the potential to meet the majority of the home’s energy requirements in a most cost-effective manner. One other key issue in the existing home sector is that of ‘hard to treat’ homes; many existing homes have already had all practical insulation measures applied, so that the only viable way of further improving the efficiency of the home is by installing an energy saving energy system such as micro-CHP or heat pumps. Yet other homes are constructed in such a way that precludes the application of cost-effective insulation measures, such as homes with solid walls, particularly listed buildings or those with attractive external features. Of course, one further compelling reason to focus on this market is that there is an established route to market with logistics, installers and marketing resources already delivering 1.5 million boilers annually. This should allow micro-CHP to make significant impact without requiring the long lead times characteristic of central plant alternatives such as nuclear power or carbon capture and sequestration (CCS); micro-CHP delivers power from day one of installation and an installation rate of 1.5 million a year represents around 1.5 GWe additional generating capacity annually, equivalent to one modern nuclear power station which would require 10 years to build and would not generate a single kilowatt hour until fully completed. Cooking 5% Lights and appliances 10%
Water heating 23%
Space heating 62%
13.2 Domestic energy consumption by end use.
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Scope of micro-CHP in existing homes The criteria for economic viability have already been discussed, outlining the requirement for payback within a reasonable period. This payback is determined largely by the thermal demand of the property in question, a minimum of 18 000 kWh being considered the threshold for lower efficiency Stirling engines. As can be seen from Fig. 13.3 which shows the distribution of gas consumption in all UK homes, this represents a target market of around 9 million homes; more electrically efficient micro-CHP technologies may become viable in smaller homes. At the extreme, products such as the CFCL SOFC unit with a heat-to-power ratio of 1:2 may be able to operate continuously to cover domestic hot water demand alone, so making it suitable for all homes with a thermal demand in excess of 2600 kWh, virtually the entire housing stock. Such products would be provided with an integral supplementary boiler to provide space heating, so it would also be able to meet the full thermal demand of even the largest homes. Objections to micro-CHP There remain those who object to the installation of current micro-CHP technologies on the basis that we should wait until more (electrically) efficient products become available. This belief is flawed; such inaction would waste the opportunity to make immediate, if modest, savings whilst we await the improved products and, as explained earlier, micro-CHP products, as gas boilers, undergo a regular replacement cycle, allowing the early products to be replaced with the enhanced performance models in due course. Still others believe we should not install any fossil-fuelled products at all, but instead await the arrival of low carbon renewable energy to fuel heat pumps,4 failing to appreciate the timescales involved and the urgent need to make the best use of our current finite gas resources until the ultimate low carbon future is attained. Another instance of the better being the enemy of the good! It is also worth considering the likely roadmap from our currently installed domestic heating systems dominated by gas-fired hydronic central heating to one in which heat pumps incrementally displace those gas boilers. There is likely to be a very significant, some would say catastrophic, shortfall in electrical generating capacity within the next decade*; this capacity shortfall would be compounded by a rapid shift to an electrified heat sector. The parallel introduction of heat pumps requiring additional electrical generating capacity and micro-CHP, which might contribute to that capacity, seems to be one effective means of achieving both decarbonised heat and avoiding *Both E.ON and EdF in the UK have indicated an anticipated shortfall in generating capacity of around 45 GWe by 2016 if they are forced to close existing coal-fired power plants to comply with the EU LCPD Directive.
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13.3 Number of UK domestic gas consumers by consumption. 75
72
69
600
450
300
150
Gas consumption band (kWh per annum) 66
63
60
57
54
51
48
45
42
39
36
33
30
27
24
21
18
15
12
0
0
000 000
000
000
000
000
000 000
000
000 000
000 000
000
000
000 000
000
000
000
000 000
000 000
000
0 000
900
600
300
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2 500 000
2 000 000
1 500 000
1 000 000
500 000
0
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overload of the electricity generation and distribution networks in the short to medium term.
13.4.3 The new-build housing market When designing and building new homes, there is the opportunity to make decisions based on the optimum combination of high performance construction and an integrated energy system. Too often, unfortunately, the innate conservativism of the construction industry and their assumptions about public aspirations, result in the major housing developers choosing to construct very poorly performing homes. They are then forced to resort to ‘addon’, sometimes tokenist (usually disproportionately costly) microgeneration solutions to comply with the increasingly stringent building regulations. Under such a scenario it is possible that micro-CHP systems will continue to be installed in traditionally constructed homes for some years although it is clear that by 2016, when zero carbon homes become mandatory, it will no longer be possible to justify micro-CHP using fossil fuels.
13.5
Small commercial buildings and other potential applications
This section considers the potential for micro-CHP units (possibly in multiple modular configurations) in small commercial buildings including residential, office, educational and other relevant applications. It can be seen from Table 13.1 that there is a much broader range of technologies which may be suitable for non-domestic applications, although the potential number of viable installations is significantly less. In addition to these stationary applications, there is also considerable global potential for micro-CHP in mobile and remote stand-alone configurations, but these are considered outside the scope of this publication. Such applications might include cabin heaters for trucks, range extenders for electric vehicles, off-grid residential and other stand-alone applications such as auxiliary power units (APU) in marine and military systems. Indeed the WhisperGen micro-CHP unit began life as a diesel-fired APU/heating system for marine applications. However, it should be recognised that the relatively low electrical efficiency of Stirling engines makes them less than ideal for applications where the primary concern is electrical generation; fuel cells are better suited to such applications.
13.5.1 UK commercial market The UK small commercial market is more fragmented than the domestic market and has a much smaller overall potential. However, there are a
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Table 13.1 Preferred technologies for micro-CHP applications
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1 kWe Stirling Existing homes
� � �
1 kWe ic engine
� �
New homes
1 kWe sofc
2 kWe sofc
5 kWe ic engine
15 kWe ic engine
50 kWe ic engine
� �
� � �
� � �
� � � � � �
� � � � � �
Sheltered Hotels
�
�
�
� �
� � �
� � �
Restaurants
�
�
�
� � �
� �
�
Schools
�
� �
� �
� �
� �
Offices
�
�
�
� �
� �
� �
� �
Emergency
�
�
�
� � �
� � �
� �
Laundrettes
�
�
�
� �
� �
Note: The stars indicate the suitability of each technology to the respective application – the more stars the better the suitability.
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number of niche markets where micro-CHP may find applications that are less sensitive to initial investment costs than the domestic market. Such areas are where there is a large demand for hot water and lighting throughout the year, prime examples being small hotels and hairdressing establishments. However, it is almost inevitable that commercial establishments which do not have a residential component are destined to have relatively low running hours simply because they are only occupied for around one-third of the day during the heating season. Schools are even less attractive due to the long holiday periods which further reduce the potential for extended operating hours, although in most cases it is possible to make use of thermal storage to extend operating hours of the CHP system during term time well beyond the occupied period by storing heat for later (or peaking) use. However, it is important to consider the likely use of the electricity generated as avoided import is clearly more valuable than the price attributed to export; it is therefore necessary to match generation to consumption as closely as possible. The following types of commercial applications, although not exhaustive, should provide an impression of the potential for micro-CHP technologies in respective building types. The summary is intended to illustrate relevant technologies, but due to the rather varied size of offices, hotels, etc., each case needs to be considered on its own merits. Applications worth consideration include the following. ∑
Hotels, where thermal demand and thus economic viability may be assessed on the basis of number of bedrooms. Each room will have a space heating demand plus a demand for sanitary hot water; on an aggregate basis it is likely that the diversified demand will be similar to a house for multiple room establishments. The prime market is probably the 13 0005 or so small hotels with 4–15 bedrooms for smaller 1–3 kWe electrical output units; the larger ICE-based units are well suited to hotels with more rooms and, although this text is focused on micro-CHP, there is clearly an overlap with larger-scale CHP plants depending on the size and thermal demands of the establishment in question. ∑ Residential and nursing homes as well as sheltered flats provide an excellent potential for micro-CHP due to the relatively high continuous thermal demands throughout the year for both space and water heating. In the majority of cases it is likely that micro-CHP units in the range of 5–15 kWe corresponding to a thermal output of 12–30 kWt would be able to achieve adequate run hours to attain a good economic return. There are in the region of 16 000 such establishments in the UK. ∑ Restaurants and pubs may have high space heating requirements, but over relatively short periods and hot water requirements tend to be light. A micro-CHP unit is unlikely to be utilised for more than 2000 hours per year, resulting in rather poor paybacks. However, if there are live-in
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tenants or rooms to let, such buildings would be prospective installations with the same characteristics as a large house. ∑ Offices, where economic viability is assessed on the basis of floor area and implied thermal demand. Although in terms of numbers, offices appear to offer a substantial market (there are over 100 000 offices with floor areas less than 100 m2) ,6 heating needs are relatively low due to the high level of internal gains from computers, lighting and other equipment. There is little requirement for water heating, giving no summer load for the CHP unit. Even in winter there is potentially a mismatch between electrical use and heating demand, since, although the electrical load in offices is fairly constant throughout the occupied period, the heating demand will be at its highest just before the start of occupancy.7 There is a further challenge to viability as offices are rarely owner occupied and there is little incentive for landlords to include micro-CHP or indeed to provide any energy efficiency measures. ∑ Emergency service buildings (police, fire and ambulance stations) which are continuously occupied and with a constant electrical demand as well as similar thermal requirements to large homes may provide a significant opportunity for micro-CHP, particularly if the unit is capable of stand-alone operation, clearly a benefit for such establishments; there are around 12 000 in the UK. ∑ Laundrettes, hairdressers and similar premises with a significant hot water and electrical demand; there are around 11 000 of these in the UK. ∑ There are also around 10 000 small schools which may benefit from micro-CHP, although it is difficult to justify the investment from the relatively short-run hours unless the building is also used for community purposes during the evenings and school holidays, or in the case of boarding schools which may effectively be considered as student halls of residence and be suitable for small-scale CHP or become part of larger CHP schemes across a campus, for example. In summary, the potential for small commercial applications suggests a market in excess of 100 000 premises. This aligns reasonably well with the gas consumption profile data which indicate a market of around 107 000 consumers with relevant gas consumptions. Depending on the size of microCHP unit under consideration, it is possible that more than one unit may be installed in some kind of cascade arrangement as shown in (Fig. 13.4). Although the larger micro-CHP units including ICE-based products can achieve high electrical efficiencies, extended service intervals and low capital cost due to their level of industrial maturity, it is possible that as production volumes of smaller units increase, their cost may become competitive with these larger units and provide a greater degree of operational flexibility. This
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13.4 Multiple EC Power 15 kWe ICE units in sheltered housing scheme.
may be particularly true for SOFC where the basic fuel cell stack may be effectively scaled by modularisation with relatively little impact on capital cost other than the balance of plant components. Assuming a replacement rate of 5% (as for domestic heating systems), and an average of two micro-CHP units per application, this represents a potential market of 5000 units per year in the small commercial sector, very much smaller than the domestic market, but conceivably less technically challenging and addressable on the basis of ‘rational’ economic decisions.
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13.6
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Advantages and limitations
13.6.1 Competing technology solutions Micro-CHP clearly has a significant role to play in a range of applications, but it is important to ensure that the appropriate technology is selected for each application. This may be Stirling engines for larger thermal demand properties, fuel cells for lower energy consuming homes and we should not be afraid to acknowledge that, in some cases, some other form of energy system altogether may be more appropriate. Micro-CHP is most cost effective where there is an adequate heat load to justify the investment and where a natural gas supply is available, but in other cases alternative technology solutions may be required. For example, practical constraints of physical size and issues with provision of mains gas supply combined with the low thermal demand of individual apartments within high rise blocks might favour community heating in preference to micro-CHP, whereas alternative technologies such as heat pumps or biomass heating are potential options in rural areas where no gas supply is available. Community heating District heating (DH), also known as community heating (CH) is relatively uncommon in the UK, with only 1% market share. This is to some extent a reflection of the high level of owner-occupation and the desire to have independent control, allied to a traditional focus in the UK on low first cost. It is also difficult to see how the conflicting demands of DH (which logically requires all homes in an area to be connected for economic viability) and a competitive market (which demands that all customers may choose their energy supplier and their energy system) can be resolved. Various studies8 have identified up to 5 million homes within areas with sufficiently high heat densities to justify district heating.* The areas identified tend to be central urban sites with high rise apartments, unsuitable for micro-CHP both from a heat loss perspective and due to the physical size and construction of the properties. Micro-CHP and DH can therefore be considered complementary rather than competing technologies, although, where practical, micro-CHP is preferable on economic and environmental grounds.9 Figure 13.5 illustrates the relative merits of various micro-CHP technologies compared with conventional gas boiler central heating plus either solar *
Note that the viability is extremely sensitive to discount rate due to the high initial cost. The 5 million figure (which includes input from the 2002 PB Power study) assumes 6%, whereas, for a rate of 9% about 400 000 and for 12%, less than 200 000 homes would be viable.
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Small and micro combined heat and power (CHP) systems Boiler (70%) PV Micro CHP (10%) Micro CHP (20%) Micro CHP (50%)
Boiler (86%) Wind Micro CHP (15%) 500 kW 0%
9000 Boiler (70%)
Annual CO2 emissions (kG)
8500 8000
Boiler (86%)
7500
We 500 k
Boiler + PV
4000
h
7000 6500
M CHP (SE)
6000 5500 M CHP (FC)
5000 4500 4000
0%
5%
10%
15%
20%
13.5 Comparative annual CO2 emissions for micro-CHP and CH/CHP.
PV or micro wind for a typical home with an annual thermal demand of 18 000 kWh and 6000 kWh electrical demand. For each of these technology combinations, the annual CO2 emissions can be seen on the vertical axis. It clearly demonstrates the environmental benefits of micro-CHP compared with these more expensive and less effective technologies, even for the lowest efficiency, Stirling engine-based micro-CHP products.10 Figure 13.5 also shows the emissions for an alternative, small (500 kWe) CHP plant connected to a small community heat network. The major disadvantage of such a system in efficiency terms is the inevitable heat distribution losses; the horizontal axis shows the assumed heat distribution losses from zero to 20%, a figure of 10% being fairly typical for a welldesigned modern system. However, the same point made above in reference to the long lead times of central electricity generating plant could also be levelled against CH as it too requires substantial, expensive and timeconsuming infrastructure investment before any useful energy is delivered at all. Whereas micro-CHP offers incremental investment risk, large-scale community heating, like nuclear, CCS, etc., requires a level of market certainty over a long period which has not so far been forthcoming. However, one major advantage of CH over micro- and small-scale CHP schemes is that, whilst the latter are largely confined to gas–fired applications, larger-scale
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systems are able to make use of both energy from waste and a wide range of alternative fuels and provide opportunities for fuel switching in response to availability. Other microgeneration technologies Depending on the space and water heating requirements of the home, the availability of a natural gas supply or alternative fuels and a host of other factors, there may be alternative microgeneration technologies which may offer alternative or complementary benefits to micro-CHP. Such technologies include biomass boilers, air and ground source heat pumps, solar thermal, micro-wind and micro-hydro. However, one of the key benefits of microCHP compared with technologies such as solar thermal, PV and microwind, is that it is a non-discretionary purchase. In other words, micro-CHP replaces an essential component of any home, namely the central heating boiler. So, as with heat pumps and biomass boilers, the investment cost of micro-CHP needs to be considered as an incremental cost compared with the alternative gas boiler, whereas the discretionary technologies need to justify their entire cost. After all you don’t need a PV system, but you do need space heating. Conventional central plant Figure 13.6 illustrates the comparative electrical and thermal efficiencies of the main micro-CHP technologies. For example, SOFC micro-CHP has an electrical efficiency of 50% and a thermal efficiency of 40% in this diagram. These are overlaid on the alternative, conventional central generating plant 90
Thermal efficiency (%)
80
Stirling engine
70
IC engine
60
PEM fuel cell
50 40
Solid oxide fuel cell
30 20 10 0 0
10
20 30 40 Electrical efficiency (%)
50
60
13.6 Comparative efficiencies of micro-CHP and conventional options.
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option providing electricity at an efficiency of 45% and 35% (representing CCGT and UK average delivered efficiencies) and heat provided by gas boilers, with state-of-the-art condensing gas boilers and typical UK gas boilers delivering efficiencies of 90% and 70%, respectively. The solid line represents the best available conventional technology, whilst the broken line is more representative of typical UK practice. Despite the variation in relative electrical and thermal efficiencies, there is no case in which microCHP performs worse than centrally generated electricity and gas boiler central heating.
13.7
Future trends
As micro-CHP enters the commercial phase, the products themselves are reaching a level of maturity which primarily requires developments to be of a ‘design for manufacture’ nature rather than addressing fundamental issues of performance and reliability. Considerations of product acceptability are also being addressed with a focus on issues such as control, interfacing with the consumer and education to ensure householders gain the most value from the operation of their micro-CHP products. The products also need to be adapted to the peculiar requirements of each market, and technology development of higher efficiency, smaller, quieter, cheaper products continues. One particularly encouraging development is the rapid progress currently being made by fuel cells which are already demonstrating exceptionally high levels of electrical efficiency, leading to the potential for application in the majority of homes. At the same time, it is becoming apparent that the implementation of micro-CHP can be accelerated by the availability of ‘enabling technologies’ such as advanced controls and metering which improve the performance within the home and which allow the true value of micro-CHP generation to be realized. Whilst there is no one technology which can overcome the challenges of UK energy policy, there is no doubt that micro-CHP can deliver a substantial and possibly the greatest individual contribution to the joint goals of eliminating fuel poverty, carbon mitigation, competitiveness and security of supply within the housing sector. There is a need to develop a range of technology solutions to meet the range of needs of different dwelling types, households and logistic constraints. It is therefore likely that new products will continue to be introduced to the market to provide householders with a range of complementary options including heat pumps, micro-wind, biomass and solar thermal technologies. Perhaps most importantly, aside from the specific developments of enhanced performance products, additional products from a variety of manufacturers and other inevitable developments of a maturing technology, there is a growing recognition that micro-CHP and other technologies must not be viewed as
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isolated components of the energy system; we must consider their synergies. The simplistic advocacy of individual technologies whether larger-scale CHP, conventional central plant solutions or alternative microgeneration technologies is giving way to an understanding that there is no one technology which is able to meet the energy demands of all consumers and applications and that we need a diversified portfolio encompassing all available low carbon, energy efficient technologies. But more than this, the technologies may not just complement one another; they may depend on one another. As we enter an era which is likely to see the emergence of very large-scale intermittent renewable generation, we need to consider the ability to arbitrage fuels, swap loads between fuel types and ideally to design systems in which, for example, the electrical output from gas-fired micro-CHP can be used to support the intermittent output of wind generation either directly through the VPP (virtual power plant) concept, or indirectly by supplying electricity to drive heat pumps when wind power is constrained. In summary, micro-CHP should be seen as a medium-term transitional technology supporting the implementation of an electrified heat sector as we move towards a zero carbon future, but may also retain a long-term, even permanent role in support of such a system.
13.8
Sources of further information and advice
∑
General information on micro-CHP and related technologies with links to manufacturers and additional resources: http://www.microchap.info ∑ General information on microgeneration technologies with links to manufacturers and additional resources: http://www.microgenerationoracle.com/index.htm ∑ Links page to government and institutional websites providing information on energy issues as well as organisations active in the field of distributed energy in general and CHP in particular: http://www.microchap.info/ LINKS.HTM
13.9
References
1 Harrison J and Redford S (2001), Potential benefits of micro CHP, Energy Saving Trust. 2 Harrison J. (2003), Micro CHP in rural areas, Renewable Energy World. 3 Department of Trade and Industry (1999), UK energy sector indicators. 4 McKay D (2009), Sustainable Energy without the hot air, UIT, Cambridge. 5 EA Technology (2000), Micro CHP – Review of emerging technologies, products, applications & markets. EA Technology report. 6 Herring H, Hardcastle R and Phillipson R (1998), Energy use and energy efficiency in UK commercial and public buildings up to the year 2000, Stationery Office Books. © Woodhead Publishing Limited, 2011
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7 Moss K (1994), Energy consumption in public and commercial buildings, BRE Information Paper, IP 16/94. 8 BRE (2003), The Potential for Community Heating in the UK, Carbon Trust. 9 Harrison J (2002), ‘Options for upgrading residential CHP’, COGEN Europe Conference Paper. 10 Harrison J (2007), ‘Micro CHP and microgeneration’, Claverton Energy Group Conference.
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Micro combined heat and power (CHP) systems for residential and small commercial buildings
J. H a r r i s o n, E.ON Engineering, UK
Abstract: The principal market for micro-CHP is as a replacement for gas boilers in the 18 million or so existing homes in the UK currently provided with gas-fired central heating systems. In addition there are a significant number of potential applications of micro-CHP in small commercial and residential buildings. In order to gain the optimum benefit from micro-CHP, it is essential to ensure that an appropriate technology is selected to integrate with the energy systems of the building. This chapter describes the key characteristics of the leading micro-CHP technologies, external and internal combustion engines and fuel cells, and how these align with the relevant applications. Key words: Stirling engine, internal combustion engine, fuel cells, microCHP.
13.1
Introduction
It is widely held that the principal market for micro-CHP is as a direct replacement of around 12 million of the 18 million gas central heating boilers, representing potential annual sales of up to 1.5 million units. This is based on the premise of simple economic payback of the investment cost of the product recoverable within an acceptable period for a 1 kWe Stirling engine micro-CHP package.1 Whilst a useful metric, it is rather simplistic, does not hold true for other micro-CHP technologies, and takes no account of potential applications in other building types. Micro-CHP derives its principal environmental and economic benefit from generating electricity as a byproduct of an existing thermal load. It replaces the gas boiler in a hydronic central heating system, producing space and water heating just as the boiler might do. It requires a primary fuel input and does not claim to be renewable, but is a low carbon and extremely energy efficient technology. As might be expected, the micro-CHP product has a higher initial cost than the boiler it replaces and must recover this cost from the value of the electricity it produces. Clearly, the more electricity that can be produced from a given thermal load, the higher the electrical output and the consequent operational income. However, it is almost invariably true 325 © Woodhead Publishing Limited, 2011
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that the higher the electrical output, the higher the capital cost so that here is both a compromise in the quest for electrical efficiency in producing a product at an appropriate cost and a natural match for different technologies to different thermal loads. The principal technologies considered relevant to mass market micro-CHP are internal combustion engines (ICE), external combustion engines (such as Stirling engines) and fuel cells. Of the latter, the high electrical efficiency and potential for low capital cost make SOFC (solid oxide fuel cells) the most promising fuel cell technology, although these are still at a relatively early stage of development and there are no commercially available products in the UK or Europe. Each of these technologies has characteristics making it more or less applicable to differing building types and thermal load profiles. However, micro-CHP is still a relatively immature technology and products continue to be developed to fulfil the requirements of various market sectors, able to compete more effectively with alternatives such as gas boilers, heat pump and larger-scale CHP technologies.
13.2
Basic issues and energy requirements
Although there are certain characteristics which make applications more or less attractive and viable, there are also some fundamental criteria which must be fulfilled to make an application viable. These include fuel availability, adequate thermal demands and an accommodating regulatory environment.
13.2.1 Economic rationale for micro-CHP The most fundamental economic factor required for viability of micro-CHP is that it must be able to recover the initial investment from the value of the electricity produced in lieu of heat from the primary fuel. It therefore holds true that the value of electricity must exceed that of heat by a significant amount. The ratio of electricity and gas prices is commonly known as ‘spark spread’ and, whilst it is generally the case that this ratio is around 3:1, reflecting in part the dominance of gas as a primary fuel input to central generating plant, it is not the case in countries such as Norway and Sweden where there is no widely available natural gas network, but an abundance of low cost hydroelectric power. In countries such as the UK, however, where retail electricity prices are around 11p/kWh and gas 3.5p/kWh, the cost of heat produced in a gas boiler with 90% efficiency is just under 4p/kWh. Thus, assuming the total efficiency of the micro-CHP unit to be equivalent to that of the gas boiler, each unit of electricity produced costs the consumer the lost opportunity cost of one unit of heat (4p), but is worth the value of displaced electricity
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(11p) assuming further that the electricity is consumed within the home and not exported. Each unit of electricity thus generates a net income of 7p; an annual production of 3000 kWh is worth up to £210. For domestic investments, this could justify a marginal capital cost of around £600 for the mass market of consumers who require a three-year payback and up to £1000 for a smaller proportion of the market who are satisfied with a payback of five years. Considering that the average UK householder moves home every seven to eight years on average, even an investment of £1500 would seem rational and early adopters of other microgeneration technologies such as PV have been prepared to invest with paybacks in excess of a century as an environmental gesture! Figure 13.1 illustrates the simplified concept of electricity generation at the expense of heat for the major micro-CHP technologies. The importance of this is that the value of each unit of electricity produced is the same (in both economic and environmental terms) regardless of the technology or its electrical efficiency; a higher electrical efficiency product will generate more units of electricity for a given heat load (and will consequently consume proportionately more gas) so that its overall income is higher, but the value per kWh of electricity is exactly the same. This belies the widely held, but erroneous view that low efficiency micro-CHP technologies are somehow environmentally inferior to higher electrical efficiency products. Indeed, the reality for currently available, high electrical efficiency ICE products is that they do not achieve the same total efficiency as gas boilers so that the saving per unit electricity generated is reduced by the cost of additional gas burned. It is of course necessary that for economic viability, the micro-CHP unit must run for enough hours each year to generate sufficient electricity to
Solid oxide fuel cell
IC engine
Heat Electricity Loss
Stirling engine
Gas boiler 0%
20%
40%
60%
80%
100%
13.1 Economic rationale for micro-CHP.
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recover the initial investment. That requires an extended heating period; not necessarily a high peak demand, but preferably a long moderate heat demand in order to make full use of the heat demand to generate electricity for at least 2500 hours annually. However, technologies currently under development such as fuel cells with very low heat-to-power ratios may be able to achieve economic viability for homes with much smaller thermal demands so that some SOFC technologies are able to operate continuous baseload to meet domestic hot water (DHW) demand with an electrical output of 1 kWe every hour of the year.
13.2.2 Fuel availability The majority of micro-CHP products are designed to operate using natural gas as a fuel input and this represents the substantial market in Europe. Although it is possible to operate micro-CHP using alternative fuels such as liquid petroleum gas (LPG), fuel oil and even biofuels,2 these are relatively expensive and do not constitute a significant potential market. It is therefore natural that access to a natural gas network is an essential requirement for mass market micro-CHP applications. In the UK the majority (18 million of the 24 million or so homes) are indeed connected to the natural gas network and are equipped with gas-fired central heating. Germany and the Netherlands also have a majority of heating systems based on hydronic natural gas central heating.
13.2.3 Regulatory environment Although not essential, it is certainly preferable that the energy market is liberalised, allowing connection of micro-CHP units to the LV electricity network and being able to provide rewards for the export of electricity to the grid. In Japan, where it is not possible to recover value for any excess generation exported to the grid, products are designed to deliver sub-optimal performance to avoid export. Although such products are made viable by the application of grants and other subsidies, the liberalised markets of many European states offer a much better prospect of sustainable economic viability.
13.2.4 Technical requirements for micro-CHP viability In addition to these criteria for economic viability, there are also physical constraints on the technologies themselves as they must be suitable for installation either in the home or in other appropriate buildings. Micro-CHP is intrinsically different from conventional CHP in that it serves the highly volatile thermal and electrical loads in individual homes
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in contrast to the more stable demands arising from the diversified loads characteristic of larger systems serving multiple homes or other building types. Therefore it is implicit that power cannot be wholly provided from CHP and that such systems will, in the majority of circumstances, be grid connected and exchange electrical energy with the network. Residential electrical loads fluctuate from below 100 We and can peak at over 20 kWe. Another complication is that residential heating systems typically operate for only around 2500 hours per year, much less than the normal CHP criteria (typically 6000 hours) for commercial viability. In addition, most manufacturers recognise it is not acceptable either practically or economically to service units more often than once annually, such as is required for gas boilers. Taken together these constraints impose severe technical demands on micro-CHP which are only just being resolved. For commercial buildings, nursing homes and similar buildings it is normal to have space allocated to a plant room which may contain bulky, relatively noisy equipment with regular access for service; in such applications ICE technology may be most appropriate particularly as this technology is able to deliver relatively high electrical efficiencies from a wide range of manufacturers. For individual homes, however, it is necessary to incorporate the micro-CHP product within the occupied space so that noise, vibration and physical bulk must be limited, as must intrusive service access requirements. It is therefore not possible to consider all technologies as equally suitable for all applications, although as a rule, technologies suitable for domestic installation are likely to be suitable also for commercial plant room applications subject to the necessary cost criteria.
13.3
Types of system for residential and small commercial buildings
Although all micro-CHP products have the common characteristic of producing heat and power from a primary fuel, fossil or otherwise, there are many different technologies acting as the ‘prime mover’, each with its own particular characteristics which make it more or less suitable for any given application. Alternatives, discussed in greater detail elsewhere in this book, include various types of engines, fuel cells, turbines and novel devices such as thermo-electric convertors.
13.3.1 External combustion engines External combustion engines separate the combustion process (which is the energy input to the engine) from the working gas, which undergoes pressure fluctuations and hence does useful work. The continuous, controlled external combustion process offers significant advantages in terms of low
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emissions, high efficiency, low noise and vibration and potentially long life and extended service intervals, although it must be conceded that these characteristics have yet to be demonstrated in the relatively immature products currently approaching commercialisation. Stirling engines have long been considered the leading micro-CHP technology, but have so far failed to reach the market in significant numbers, although it is expected that both WhisperGen and Baxi will launch their mass-produced 1 kWe products on the UK market during 2010 through the energy companies E.ON and British Gas, respectively. Both products have an electrical output around 1 kWe and electrical efficiencies approaching 15% and with overall efficiencies similar to those of gas boilers. For a typical UK home they would be expected to generate around 3000 kWh of low carbon electricity with a value of up to £200 if all the generated power were consumed on site and an additional £300 from the proposed FIT (feed-in tariff) subsidy. However, it should be noted that promoters of these technologies recommend their installation in larger homes with higher than average thermal demands in order to maximise the income from electricity generation and minimise payback; such homes, with an annual thermal demand of greater than 18 000 kWh constitute around half of the gas centrally heated homes in the UK.
13.3.2 Internal combustion engines Internal combustion engines inject fuel and air into the cylinders where combustion occurs. The resulting temperature and pressure changes of the fuel/air mixture (which is also the working gas) act on the piston to produce useful work. This is mature technology, able to draw on extensive experience in both stationary and automotive applications, although the characteristic high emissions, noise and vibration as well as high service requirements inherent in this technology, raise significant challenges for micro-CHP applications. However, current products available from as little as 1 kWe and with a range of outputs up to 50 kWe (the upper limit of microgeneration*) seem to have substantially overcome each of these challenges and there are around 100 000 Honda Ecowill (1 kWe) units installed in Japanese homes and over 10 000 Baxi Dachs (5 kWe) units installed in large homes and similar buildings in Europe. With an electrical efficiency in excess of 25%, the electrical output of these products in many homes can be quite substantial, possibly as much as 4000 kWh per year and generating an income of around £240† (excluding *As defined by the UK Government in the Climate Change and Sustainable Energy Act 2007. † As the total efficiency of the leading current ICE product is significantly less than that of a gas boiler, the higher production cost of each unit of electricity results in a net income of only 6p.
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subsidies which could add another £400 in the UK). The relatively low heat to power ratio of 3:1 requires the inclusion of a supplementary burner in the micro-CHP package, but makes installation in lower thermal demand homes economically viable.
13.3.3 Fuel cells In a fuel cell, the chemical energy within the fuel is converted directly into electricity (with byproducts of heat and water) without any mechanical drive or generator. Generally speaking they have a much higher electrical conversion efficiency than other technologies, particularly in the case of SOFC technologies, but are relatively inflexible in performance, requiring more or less continuous operation to avoid thermal cycling and the consequent induced mechanical stresses. The leading SOFC technology with an electrical efficiency of 60% and a heat to power ratio of 1:2 operates to provide DHW (domestic hot water) throughout the year, any requirement for space heating being met by the supplementary burner included within the package. In theory, this product would be technically suitable for every gas heated home in the country, although it is a relatively bulky product, requiring a substantial thermal store to capture the ‘waste’ thermal energy in the form of hot water and is thus probably best suited to homes with the necessary available space.
13.3.4 Other novel technologies There are numerous experimental technologies which may at some future date result in useable products. These include thermo-ionic and thermoelectric technologies which utilise temperature difference acting on metals or semi-conductors to produce electricity together with thermo-photovoltaic units which convert the radiant energy emitted by the burner to produce electricity using infra-red sensitive PV cells.
13.4
Domestic applications for micro combined heat and power (CHP)
There is currently a limited number of prime mover technologies on which micro-CHP systems suitable for individual homes can be based. Given the relative technical immaturity of micro-CHP, it is not always currently possible to identify a micro-CHP product ideally matched to the specific application. The mass market for micro-CHP is for stationary, on-grid applications, primarily in domestic buildings ranging from individual detached houses to multi-storey, apartment blocks. It should be acknowledged that, for some of these applications, particularly very high density apartments, some form
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of communal CHP system may be more appropriate from an economic and environmental perspective, although even in such instances, the desire for independence and personal control may make micro-CHP the preferred option from the user’s point of view. As noted earlier, by far the largest potential market for micro-CHP is for installation in individual homes which currently have gas-fuelled hydronic central heating systems. The technology is particularly relevant to provide heating in larger, less energy efficient homes where all practical, cost effective measures have already been taken to reduce energy cost, but where a significant thermal demand remains, so-called ‘hard to treat’ homes.
13.4.1 Complementary applications of micro-CHP in housing Currently available micro-CHP products simply provide space and water heating together with electricity. This limits their operating hours to periods when space heating is required in winter plus an additional limited duration outside the heating season, perhaps an hour or so daily, when there is still a demand for hot water. Some commentators have suggested measures to enhance the performance of their micro-CHP concepts by extending the potential operating hours to meet other heat-based loads. In theory, opportunities do exist for more complex, hybrid packages including cooling to make use of the heat output and thus generate more electricity outside the heating season. However, in practice, absorption cooling has yet to be demonstrated successfully at this scale. Still others propose the application of the generated electricity to power a vapour compression heat pump to maximise the thermal output of the unit, either on a continuous basis or for peaking purposes; this same principle could be applied as a fuel arbitrage measure in the longer term when the dominant domestic heating technology may be air source heat pumps. During periods of limited electricity availability from intermittent renewable sources, effectively the micro-CHP unit becomes a component of a bivalent heat pump heating package.
13.4.2 The existing homes market It has already been explained that the micro-CHP technologies currently under consideration are assumed to replace the boiler in a hydronic system. The majority of UK homes (18 million out of 25 million) are equipped with such central heating systems, as are those of the Netherlands and Germany, generally assumed to be the other two key European markets. At the same time the majority of the 1.5 million gas boilers sold each year are for replacement of boilers in existing systems when they reach the end of their useful life, typically after 10–15 years. It therefore seems logical to focus
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marketing activities on this substantial market provided that the micro-CHP technology matches the thermal characteristics of the home. Not only do existing homes tend to have relatively higher thermal demands than newer, better insulated homes, but the heat-to-power ratio also aligns well with that of Stirling-based systems currently being introduced to the market. Around 85% of energy use within the home is for space and water heating, the remaining 15% is electricity3, as shown in Fig. 13.2. Thus the output of the micro-CHP unit, matching as it does this ratio, has the potential to meet the majority of the home’s energy requirements in a most cost-effective manner. One other key issue in the existing home sector is that of ‘hard to treat’ homes; many existing homes have already had all practical insulation measures applied, so that the only viable way of further improving the efficiency of the home is by installing an energy saving energy system such as micro-CHP or heat pumps. Yet other homes are constructed in such a way that precludes the application of cost-effective insulation measures, such as homes with solid walls, particularly listed buildings or those with attractive external features. Of course, one further compelling reason to focus on this market is that there is an established route to market with logistics, installers and marketing resources already delivering 1.5 million boilers annually. This should allow micro-CHP to make significant impact without requiring the long lead times characteristic of central plant alternatives such as nuclear power or carbon capture and sequestration (CCS); micro-CHP delivers power from day one of installation and an installation rate of 1.5 million a year represents around 1.5 GWe additional generating capacity annually, equivalent to one modern nuclear power station which would require 10 years to build and would not generate a single kilowatt hour until fully completed. Cooking 5% Lights and appliances 10%
Water heating 23%
Space heating 62%
13.2 Domestic energy consumption by end use.
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Scope of micro-CHP in existing homes The criteria for economic viability have already been discussed, outlining the requirement for payback within a reasonable period. This payback is determined largely by the thermal demand of the property in question, a minimum of 18 000 kWh being considered the threshold for lower efficiency Stirling engines. As can be seen from Fig. 13.3 which shows the distribution of gas consumption in all UK homes, this represents a target market of around 9 million homes; more electrically efficient micro-CHP technologies may become viable in smaller homes. At the extreme, products such as the CFCL SOFC unit with a heat-to-power ratio of 1:2 may be able to operate continuously to cover domestic hot water demand alone, so making it suitable for all homes with a thermal demand in excess of 2600 kWh, virtually the entire housing stock. Such products would be provided with an integral supplementary boiler to provide space heating, so it would also be able to meet the full thermal demand of even the largest homes. Objections to micro-CHP There remain those who object to the installation of current micro-CHP technologies on the basis that we should wait until more (electrically) efficient products become available. This belief is flawed; such inaction would waste the opportunity to make immediate, if modest, savings whilst we await the improved products and, as explained earlier, micro-CHP products, as gas boilers, undergo a regular replacement cycle, allowing the early products to be replaced with the enhanced performance models in due course. Still others believe we should not install any fossil-fuelled products at all, but instead await the arrival of low carbon renewable energy to fuel heat pumps,4 failing to appreciate the timescales involved and the urgent need to make the best use of our current finite gas resources until the ultimate low carbon future is attained. Another instance of the better being the enemy of the good! It is also worth considering the likely roadmap from our currently installed domestic heating systems dominated by gas-fired hydronic central heating to one in which heat pumps incrementally displace those gas boilers. There is likely to be a very significant, some would say catastrophic, shortfall in electrical generating capacity within the next decade*; this capacity shortfall would be compounded by a rapid shift to an electrified heat sector. The parallel introduction of heat pumps requiring additional electrical generating capacity and micro-CHP, which might contribute to that capacity, seems to be one effective means of achieving both decarbonised heat and avoiding *Both E.ON and EdF in the UK have indicated an anticipated shortfall in generating capacity of around 45 GWe by 2016 if they are forced to close existing coal-fired power plants to comply with the EU LCPD Directive.
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13.3 Number of UK domestic gas consumers by consumption. 75
72
69
600
450
300
150
Gas consumption band (kWh per annum) 66
63
60
57
54
51
48
45
42
39
36
33
30
27
24
21
18
15
12
0
0
000 000
000
000
000
000
000 000
000
000 000
000 000
000
000
000 000
000
000
000
000 000
000 000
000
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overload of the electricity generation and distribution networks in the short to medium term.
13.4.3 The new-build housing market When designing and building new homes, there is the opportunity to make decisions based on the optimum combination of high performance construction and an integrated energy system. Too often, unfortunately, the innate conservativism of the construction industry and their assumptions about public aspirations, result in the major housing developers choosing to construct very poorly performing homes. They are then forced to resort to ‘addon’, sometimes tokenist (usually disproportionately costly) microgeneration solutions to comply with the increasingly stringent building regulations. Under such a scenario it is possible that micro-CHP systems will continue to be installed in traditionally constructed homes for some years although it is clear that by 2016, when zero carbon homes become mandatory, it will no longer be possible to justify micro-CHP using fossil fuels.
13.5
Small commercial buildings and other potential applications
This section considers the potential for micro-CHP units (possibly in multiple modular configurations) in small commercial buildings including residential, office, educational and other relevant applications. It can be seen from Table 13.1 that there is a much broader range of technologies which may be suitable for non-domestic applications, although the potential number of viable installations is significantly less. In addition to these stationary applications, there is also considerable global potential for micro-CHP in mobile and remote stand-alone configurations, but these are considered outside the scope of this publication. Such applications might include cabin heaters for trucks, range extenders for electric vehicles, off-grid residential and other stand-alone applications such as auxiliary power units (APU) in marine and military systems. Indeed the WhisperGen micro-CHP unit began life as a diesel-fired APU/heating system for marine applications. However, it should be recognised that the relatively low electrical efficiency of Stirling engines makes them less than ideal for applications where the primary concern is electrical generation; fuel cells are better suited to such applications.
13.5.1 UK commercial market The UK small commercial market is more fragmented than the domestic market and has a much smaller overall potential. However, there are a
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Table 13.1 Preferred technologies for micro-CHP applications
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1 kWe Stirling Existing homes
� � �
1 kWe ic engine
� �
New homes
1 kWe sofc
2 kWe sofc
5 kWe ic engine
15 kWe ic engine
50 kWe ic engine
� �
� � �
� � �
� � � � � �
� � � � � �
Sheltered Hotels
�
�
�
� �
� � �
� � �
Restaurants
�
�
�
� � �
� �
�
Schools
�
� �
� �
� �
� �
Offices
�
�
�
� �
� �
� �
� �
Emergency
�
�
�
� � �
� � �
� �
Laundrettes
�
�
�
� �
� �
Note: The stars indicate the suitability of each technology to the respective application – the more stars the better the suitability.
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number of niche markets where micro-CHP may find applications that are less sensitive to initial investment costs than the domestic market. Such areas are where there is a large demand for hot water and lighting throughout the year, prime examples being small hotels and hairdressing establishments. However, it is almost inevitable that commercial establishments which do not have a residential component are destined to have relatively low running hours simply because they are only occupied for around one-third of the day during the heating season. Schools are even less attractive due to the long holiday periods which further reduce the potential for extended operating hours, although in most cases it is possible to make use of thermal storage to extend operating hours of the CHP system during term time well beyond the occupied period by storing heat for later (or peaking) use. However, it is important to consider the likely use of the electricity generated as avoided import is clearly more valuable than the price attributed to export; it is therefore necessary to match generation to consumption as closely as possible. The following types of commercial applications, although not exhaustive, should provide an impression of the potential for micro-CHP technologies in respective building types. The summary is intended to illustrate relevant technologies, but due to the rather varied size of offices, hotels, etc., each case needs to be considered on its own merits. Applications worth consideration include the following. ∑
Hotels, where thermal demand and thus economic viability may be assessed on the basis of number of bedrooms. Each room will have a space heating demand plus a demand for sanitary hot water; on an aggregate basis it is likely that the diversified demand will be similar to a house for multiple room establishments. The prime market is probably the 13 0005 or so small hotels with 4–15 bedrooms for smaller 1–3 kWe electrical output units; the larger ICE-based units are well suited to hotels with more rooms and, although this text is focused on micro-CHP, there is clearly an overlap with larger-scale CHP plants depending on the size and thermal demands of the establishment in question. ∑ Residential and nursing homes as well as sheltered flats provide an excellent potential for micro-CHP due to the relatively high continuous thermal demands throughout the year for both space and water heating. In the majority of cases it is likely that micro-CHP units in the range of 5–15 kWe corresponding to a thermal output of 12–30 kWt would be able to achieve adequate run hours to attain a good economic return. There are in the region of 16 000 such establishments in the UK. ∑ Restaurants and pubs may have high space heating requirements, but over relatively short periods and hot water requirements tend to be light. A micro-CHP unit is unlikely to be utilised for more than 2000 hours per year, resulting in rather poor paybacks. However, if there are live-in
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tenants or rooms to let, such buildings would be prospective installations with the same characteristics as a large house. ∑ Offices, where economic viability is assessed on the basis of floor area and implied thermal demand. Although in terms of numbers, offices appear to offer a substantial market (there are over 100 000 offices with floor areas less than 100 m2) ,6 heating needs are relatively low due to the high level of internal gains from computers, lighting and other equipment. There is little requirement for water heating, giving no summer load for the CHP unit. Even in winter there is potentially a mismatch between electrical use and heating demand, since, although the electrical load in offices is fairly constant throughout the occupied period, the heating demand will be at its highest just before the start of occupancy.7 There is a further challenge to viability as offices are rarely owner occupied and there is little incentive for landlords to include micro-CHP or indeed to provide any energy efficiency measures. ∑ Emergency service buildings (police, fire and ambulance stations) which are continuously occupied and with a constant electrical demand as well as similar thermal requirements to large homes may provide a significant opportunity for micro-CHP, particularly if the unit is capable of stand-alone operation, clearly a benefit for such establishments; there are around 12 000 in the UK. ∑ Laundrettes, hairdressers and similar premises with a significant hot water and electrical demand; there are around 11 000 of these in the UK. ∑ There are also around 10 000 small schools which may benefit from micro-CHP, although it is difficult to justify the investment from the relatively short-run hours unless the building is also used for community purposes during the evenings and school holidays, or in the case of boarding schools which may effectively be considered as student halls of residence and be suitable for small-scale CHP or become part of larger CHP schemes across a campus, for example. In summary, the potential for small commercial applications suggests a market in excess of 100 000 premises. This aligns reasonably well with the gas consumption profile data which indicate a market of around 107 000 consumers with relevant gas consumptions. Depending on the size of microCHP unit under consideration, it is possible that more than one unit may be installed in some kind of cascade arrangement as shown in (Fig. 13.4). Although the larger micro-CHP units including ICE-based products can achieve high electrical efficiencies, extended service intervals and low capital cost due to their level of industrial maturity, it is possible that as production volumes of smaller units increase, their cost may become competitive with these larger units and provide a greater degree of operational flexibility. This
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13.4 Multiple EC Power 15 kWe ICE units in sheltered housing scheme.
may be particularly true for SOFC where the basic fuel cell stack may be effectively scaled by modularisation with relatively little impact on capital cost other than the balance of plant components. Assuming a replacement rate of 5% (as for domestic heating systems), and an average of two micro-CHP units per application, this represents a potential market of 5000 units per year in the small commercial sector, very much smaller than the domestic market, but conceivably less technically challenging and addressable on the basis of ‘rational’ economic decisions.
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13.6
341
Advantages and limitations
13.6.1 Competing technology solutions Micro-CHP clearly has a significant role to play in a range of applications, but it is important to ensure that the appropriate technology is selected for each application. This may be Stirling engines for larger thermal demand properties, fuel cells for lower energy consuming homes and we should not be afraid to acknowledge that, in some cases, some other form of energy system altogether may be more appropriate. Micro-CHP is most cost effective where there is an adequate heat load to justify the investment and where a natural gas supply is available, but in other cases alternative technology solutions may be required. For example, practical constraints of physical size and issues with provision of mains gas supply combined with the low thermal demand of individual apartments within high rise blocks might favour community heating in preference to micro-CHP, whereas alternative technologies such as heat pumps or biomass heating are potential options in rural areas where no gas supply is available. Community heating District heating (DH), also known as community heating (CH) is relatively uncommon in the UK, with only 1% market share. This is to some extent a reflection of the high level of owner-occupation and the desire to have independent control, allied to a traditional focus in the UK on low first cost. It is also difficult to see how the conflicting demands of DH (which logically requires all homes in an area to be connected for economic viability) and a competitive market (which demands that all customers may choose their energy supplier and their energy system) can be resolved. Various studies8 have identified up to 5 million homes within areas with sufficiently high heat densities to justify district heating.* The areas identified tend to be central urban sites with high rise apartments, unsuitable for micro-CHP both from a heat loss perspective and due to the physical size and construction of the properties. Micro-CHP and DH can therefore be considered complementary rather than competing technologies, although, where practical, micro-CHP is preferable on economic and environmental grounds.9 Figure 13.5 illustrates the relative merits of various micro-CHP technologies compared with conventional gas boiler central heating plus either solar *
Note that the viability is extremely sensitive to discount rate due to the high initial cost. The 5 million figure (which includes input from the 2002 PB Power study) assumes 6%, whereas, for a rate of 9% about 400 000 and for 12%, less than 200 000 homes would be viable.
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Small and micro combined heat and power (CHP) systems Boiler (70%) PV Micro CHP (10%) Micro CHP (20%) Micro CHP (50%)
Boiler (86%) Wind Micro CHP (15%) 500 kW 0%
9000 Boiler (70%)
Annual CO2 emissions (kG)
8500 8000
Boiler (86%)
7500
We 500 k
Boiler + PV
4000
h
7000 6500
M CHP (SE)
6000 5500 M CHP (FC)
5000 4500 4000
0%
5%
10%
15%
20%
13.5 Comparative annual CO2 emissions for micro-CHP and CH/CHP.
PV or micro wind for a typical home with an annual thermal demand of 18 000 kWh and 6000 kWh electrical demand. For each of these technology combinations, the annual CO2 emissions can be seen on the vertical axis. It clearly demonstrates the environmental benefits of micro-CHP compared with these more expensive and less effective technologies, even for the lowest efficiency, Stirling engine-based micro-CHP products.10 Figure 13.5 also shows the emissions for an alternative, small (500 kWe) CHP plant connected to a small community heat network. The major disadvantage of such a system in efficiency terms is the inevitable heat distribution losses; the horizontal axis shows the assumed heat distribution losses from zero to 20%, a figure of 10% being fairly typical for a welldesigned modern system. However, the same point made above in reference to the long lead times of central electricity generating plant could also be levelled against CH as it too requires substantial, expensive and timeconsuming infrastructure investment before any useful energy is delivered at all. Whereas micro-CHP offers incremental investment risk, large-scale community heating, like nuclear, CCS, etc., requires a level of market certainty over a long period which has not so far been forthcoming. However, one major advantage of CH over micro- and small-scale CHP schemes is that, whilst the latter are largely confined to gas–fired applications, larger-scale
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systems are able to make use of both energy from waste and a wide range of alternative fuels and provide opportunities for fuel switching in response to availability. Other microgeneration technologies Depending on the space and water heating requirements of the home, the availability of a natural gas supply or alternative fuels and a host of other factors, there may be alternative microgeneration technologies which may offer alternative or complementary benefits to micro-CHP. Such technologies include biomass boilers, air and ground source heat pumps, solar thermal, micro-wind and micro-hydro. However, one of the key benefits of microCHP compared with technologies such as solar thermal, PV and microwind, is that it is a non-discretionary purchase. In other words, micro-CHP replaces an essential component of any home, namely the central heating boiler. So, as with heat pumps and biomass boilers, the investment cost of micro-CHP needs to be considered as an incremental cost compared with the alternative gas boiler, whereas the discretionary technologies need to justify their entire cost. After all you don’t need a PV system, but you do need space heating. Conventional central plant Figure 13.6 illustrates the comparative electrical and thermal efficiencies of the main micro-CHP technologies. For example, SOFC micro-CHP has an electrical efficiency of 50% and a thermal efficiency of 40% in this diagram. These are overlaid on the alternative, conventional central generating plant 90
Thermal efficiency (%)
80
Stirling engine
70
IC engine
60
PEM fuel cell
50 40
Solid oxide fuel cell
30 20 10 0 0
10
20 30 40 Electrical efficiency (%)
50
60
13.6 Comparative efficiencies of micro-CHP and conventional options.
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option providing electricity at an efficiency of 45% and 35% (representing CCGT and UK average delivered efficiencies) and heat provided by gas boilers, with state-of-the-art condensing gas boilers and typical UK gas boilers delivering efficiencies of 90% and 70%, respectively. The solid line represents the best available conventional technology, whilst the broken line is more representative of typical UK practice. Despite the variation in relative electrical and thermal efficiencies, there is no case in which microCHP performs worse than centrally generated electricity and gas boiler central heating.
13.7
Future trends
As micro-CHP enters the commercial phase, the products themselves are reaching a level of maturity which primarily requires developments to be of a ‘design for manufacture’ nature rather than addressing fundamental issues of performance and reliability. Considerations of product acceptability are also being addressed with a focus on issues such as control, interfacing with the consumer and education to ensure householders gain the most value from the operation of their micro-CHP products. The products also need to be adapted to the peculiar requirements of each market, and technology development of higher efficiency, smaller, quieter, cheaper products continues. One particularly encouraging development is the rapid progress currently being made by fuel cells which are already demonstrating exceptionally high levels of electrical efficiency, leading to the potential for application in the majority of homes. At the same time, it is becoming apparent that the implementation of micro-CHP can be accelerated by the availability of ‘enabling technologies’ such as advanced controls and metering which improve the performance within the home and which allow the true value of micro-CHP generation to be realized. Whilst there is no one technology which can overcome the challenges of UK energy policy, there is no doubt that micro-CHP can deliver a substantial and possibly the greatest individual contribution to the joint goals of eliminating fuel poverty, carbon mitigation, competitiveness and security of supply within the housing sector. There is a need to develop a range of technology solutions to meet the range of needs of different dwelling types, households and logistic constraints. It is therefore likely that new products will continue to be introduced to the market to provide householders with a range of complementary options including heat pumps, micro-wind, biomass and solar thermal technologies. Perhaps most importantly, aside from the specific developments of enhanced performance products, additional products from a variety of manufacturers and other inevitable developments of a maturing technology, there is a growing recognition that micro-CHP and other technologies must not be viewed as
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isolated components of the energy system; we must consider their synergies. The simplistic advocacy of individual technologies whether larger-scale CHP, conventional central plant solutions or alternative microgeneration technologies is giving way to an understanding that there is no one technology which is able to meet the energy demands of all consumers and applications and that we need a diversified portfolio encompassing all available low carbon, energy efficient technologies. But more than this, the technologies may not just complement one another; they may depend on one another. As we enter an era which is likely to see the emergence of very large-scale intermittent renewable generation, we need to consider the ability to arbitrage fuels, swap loads between fuel types and ideally to design systems in which, for example, the electrical output from gas-fired micro-CHP can be used to support the intermittent output of wind generation either directly through the VPP (virtual power plant) concept, or indirectly by supplying electricity to drive heat pumps when wind power is constrained. In summary, micro-CHP should be seen as a medium-term transitional technology supporting the implementation of an electrified heat sector as we move towards a zero carbon future, but may also retain a long-term, even permanent role in support of such a system.
13.8
Sources of further information and advice
∑
General information on micro-CHP and related technologies with links to manufacturers and additional resources: http://www.microchap.info ∑ General information on microgeneration technologies with links to manufacturers and additional resources: http://www.microgenerationoracle.com/index.htm ∑ Links page to government and institutional websites providing information on energy issues as well as organisations active in the field of distributed energy in general and CHP in particular: http://www.microchap.info/ LINKS.HTM
13.9
References
1 Harrison J and Redford S (2001), Potential benefits of micro CHP, Energy Saving Trust. 2 Harrison J. (2003), Micro CHP in rural areas, Renewable Energy World. 3 Department of Trade and Industry (1999), UK energy sector indicators. 4 McKay D (2009), Sustainable Energy without the hot air, UIT, Cambridge. 5 EA Technology (2000), Micro CHP – Review of emerging technologies, products, applications & markets. EA Technology report. 6 Herring H, Hardcastle R and Phillipson R (1998), Energy use and energy efficiency in UK commercial and public buildings up to the year 2000, Stationery Office Books. © Woodhead Publishing Limited, 2011
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7 Moss K (1994), Energy consumption in public and commercial buildings, BRE Information Paper, IP 16/94. 8 BRE (2003), The Potential for Community Heating in the UK, Carbon Trust. 9 Harrison J (2002), ‘Options for upgrading residential CHP’, COGEN Europe Conference Paper. 10 Harrison J (2007), ‘Micro CHP and microgeneration’, Claverton Energy Group Conference.
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