Micro combined heat and power or micro-CHP is an extension of the now well established idea of cogeneration to the single/multi family home or small office building.
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Micro-chp is defined by the EC as being of less than 50 kW electrical power output. In the majority of energy applications, energy is required in multiple forms. These energy forms typically include some combination of: heating, ventilation, and air conditioning, mechanical energy and electric power. Often, these additional forms of energy are produced by a heat engine, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the second law of thermodynamics, therefore a heat engine will always produce a surplus of low-temperature heat. This is commonly referred to as waste heat or "secondary heat", or "low-grade heat". This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike electricity or fuel energy. By transporting fuel, however, the "waste heat" is essentially being transported along with the fuel, before the waste heat is actually produced.
To make efficient use of energy, the "waste heat" must be used purposefully. Since it is practical to transport electricity, but not always practical to transport waste heat, an energy efficient system must generate electricity near locations where the waste heat can be put to good use. This is known as a combined heat and power (CHP) system, or "cogeneration".
In a central power plant, the supply of "waste heat" may exceed the local heat demand. In such cases, if it is not desirable to reduce the power production, the excess waste heat must be disposed in e.g. cooling towers or sea cooling without being used. A way to avoid excess waste heat is to reduce the fuel input to the CHP plant, reducing both the heat and power output to balance the heat demand. In doing this, the power production is limited by the heat demand.
CHP systems are able to increase the total energy utilization of primary energy sources, such as fuel and concentrated solar thermal energy. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of fuels, particularly oil-based fuels, and due to environmental concerns, particularly climate change.
In a traditional power plant delivering electricity to consumers, about 30% of the heat content of the primary heat energy source, such as biomass, coal, solar thermal, natural gas, petroleum or uranium, reaches the consumer, although the efficiency can be 20% for very old plants and 45% for newer gas plants. In contrast, a CHP system converts 15%–42% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. In total, as much as 90% of the heat from the primary energy source goes to useful purposes when heat production does not exceed the demand.
CHP systems have benefited the industrial sector since the beginning of the industrial revolution. For three decades, these larger CHP systems were more economically justifiable than micro-CHP, due to the economy of scale. After the year 2000, micro-CHP has become cost effective in many markets around the world, due to rising energy costs. The development of micro-CHP systems has also been facilitated by recent technological developments of small heat engines. This includes improved performance and/or cost-effectiveness of fuel cells, Stirling engines, steam engines, gas turbines, diesel engines and Otto engines.
In many cases industrial CHP systems primarily generate electricity and heat is a by-product; micro-CHP systems in homes or small commercial buildings are controlled by heat-demand, delivering electricity as the by-product. When used primarily for heat in circumstances of fluctuating electrical demand, micro-CHP systems will often generate more electricity than is instantly being demanded.
To date, micro-CHP systems achieve much of their savings, and thus attractiveness to consumers, through a "generate-and-resell" or net metering model wherein home-generated power exceeding the instantaneous in-home needs is sold back to the electrical utility. This system is efficient because the energy used is distributed and used instantaneously over the electrical grid. The main losses are in the transmission from the source to the consumer which will typically be less than losses incurred by storing energy locally or generating power at less than the peak efficiency of the micro-CHP system. So, from a purely technical standpoint dynamic demand management and net-metering are very efficient.
Another positive to net-metering is the fact that it is fairly easy to configure. The user's electrical meter is simply able to record electrical power exiting as well as entering the home or business. As such, it records the net amount of power entering the home. For a grid with relatively few micro-CHP users, no design changes to the electrical grid need be made. Additionally, in the United States, federal and now many state regulations require utility operators to compensate anyone adding power to the grid. From the standpoint of grid operator, these points present operational and technical as well as administrative burdens. As a consequence, most grid operators compensate non-utility power-contributors at less than or equal to the rate they charge their customers. While this compensation scheme may seem almost fair at first glance, it only represents the consumer’s cost-savings of not purchasing utility power versus the true cost of generation and operation to the micro-CHP operator. Thus from the standpoint of micro-CHP operators, net-metering is not ideal.
While net-metering is a very efficient mechanism for using excess energy generated by a micro-CHP system, it is not without its detractors. Of the detractors' main points, the first to consider is that while the main generating source on the electrical grid is a large commercial generator, net-metering generators "spill" power to the smart grid in a haphazard and unpredictable fashion. However, the effect is negligible if there are only a small percentage of customers generating electricity and each of them generates a relatively small amount of electricity. When turning on an oven or space heater, about the same amount of electricity is drawn from the grid as a home generator puts out. If the percentage of homes with generating systems becomes large, then the effect on the grid may become significant. Coordination among the generating systems in homes and the rest of the grid may be necessary for reliable operation and to prevent damage to the grid.
In an evaluation from 2008 by Claverton Energy Group, Stirling engined micro CHP was deemed the most cost effective of the various microgeneration technologies in abating carbon in the UK.[1]
Micro-CHP engine systems are currently based on several different technologies:
The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly, it can be inexpensive, it is available in most areas and is easily transported through pipelines, which already exist for many homes. Natural gas is suitable for internal combustion engines, such as Otto engine and gas turbine systems. Gas turbines are used in many small systems due to their high efficiency, small size, clean combustion, durability and low maintenance requirements. Gas turbines designed with foil bearings and air-cooling, operate without lubricating oil or coolants. The waste heat of gas turbines is mostly in the exhaust, whereas the waste heat of reciprocating internal combustion engines, is split between the exhaust and cooling system.
The future of combined heat and power, particularly for homes and small businesses, will continue to be affected by the price of fuel, including natural gas. As fuel prices continue to climb, this will make the economics more favorable for energy conservation measures, and more efficient energy use, including CHP and micro-CHP.
There are many types of fuels and sources of heat that may be considered for micro-CHP. The properties of these sources vary in terms of system cost, heat cost, environmental effects, convenience, ease of transportation and storage, system maintenance, and system life. Some of the heat sources and fuels that are being considered for use with micro-CHP include: biomass, LPG, vegetable oil (such as rapeseed oil), woodgas, solar thermal, and natural gas, as well as multi-fuel systems. (Nuclear power is hazardous at small scales, due to radiation risks, so it is generally not viable for micro-CHP.) The energy sources with the lowest emissions of particulates and net-carbon dioxide, include solar power, biomass (with two-stage gasification into biogas), and natural gas.
External combustion engines can run on any high-temperature heat source. These engines include the Stirling engine, hot "gas" turbocharger, steam engine. Both range from 10%-20% efficiency, and as of 2008, small quantities are in production for micro-CHP products.
Other possibilities include the Organic Rankine cycle, which operates at lower temperatures and pressures using low-grade heat sources. The primary advantage to this is that the equipment is essentially an air-conditioning or refrigeration unit operating as an engine, whereby the piping and other components need not be designed for extreme temperatures and pressures, reducing cost and complexity. Electrical efficiency suffers, but it is presumed that such a system would be utilizing waste heat or a heat source such as a wood stove or gas boiler that would exist anyways for purposes of space heating.
Fuel cells generate electricity and heat as a by product. The advantages over stirling CHP are no moving parts, less maintenance, and quieter operation. The surplus electricity can be delivered back to the grid.[2]
As an example, a PEMFC fuel cell based micro-CHP has an electrical efficiency of 37% LHV and 33% HHV and a heat recovery efficiency of 52% LHV and 47% HHV with a service life of 40,000 hours or 4000 start/stop cycles which is equal to 10 year use.
United States Department of Energy (DOE) Technical Targets: 1–10 kW residential combined heat and power fuel cells operating on natural gas.[3]
| Type | 2008 Status | 2012 | 2015 | 2020 |
|---|---|---|---|---|
| Electrical efficiency at rated power2 | 34% | 40% | 42.5% | 45% |
| CHP energy efficiency3 | 80% | 85% | 87.5% | 90% |
| Factory cost4 | $750/kW | $650/kW | $550/kW | $450/kW |
| Transient response (10%–90% rated power) | 5 min | 4 min | 3 min | 2 min |
| Start-up time from 20 °C ambient temperature | 60 min | 45 min | 30 min | 20 min |
| Degradation with cycling5 | < 2%/1000 h | 0.7%/1000 h | 0.5%/1000 h | 0.3%/1000 h |
| Operating lifetime6 | 6,000 h | 30,000 h | 40,000 h | 60,000 h |
| System availability | 97% | 97.5% | 98% | 99% |
1Standard utility natural gas delivered at typical residential distribution line pressures. 2Regulated AC net/lower heating value of fuel. 3Only heat available at 80 °C or higher is included in CHP energy efficiency calculation. 4Cost includes materials and labor costs to produce stack, plus any balance of plant necessary for stack operation. Cost defined at 50,000 unit/year production (250 MW in 5 kW modules). 5Based on operating cycle to be released in 2010. 6Time until >20% net power degradation.
Thermoelectric generators operating on the Seebeck Effect show promise due to their total absence of moving parts. Efficiency, however, is the major concern as most thermoelectric devices fail to achieve 5% efficiency even with high temperature differences.
This can be achieved by Photovoltaic thermal hybrid solar collector, another option is Concentrated photovoltaics and thermal (CPVT), also sometimes called combined heat and power solar (CHAPS), is a cogeneration technology used in concentrated photovoltaics that produce both electricity and heat in the same module. The heat may be employed in district heating, water heating and air conditioning, desalination or process heat.
CPVT systems are currently in production in Europe,[4] with Zenith Solar developing CPVT systems with a claimed efficiency of 72%.[5]
Sopogy produces a micro Concentrated solar system (microCSP) system based on parabolic trough which can be installed above building or homes, the heat can be used for water heating or solar air conditioning, a steam turbine can also be installed to produce electricity.
The recent development of small scale CHP systems has provided the opportunity for in-house power backup of residential-scale photovoltaic (PV) arrays.[6] The results of a recent study show that a PV+CHP hybrid system not only has the potential to radically reduce energy waste in the status quo electrical and heating systems, but it also enables the share of solar PV to be expanded by about a factor of five.[7] In some regions, in order to reduce waste from excess heat, an absorption chiller has been proposed to utilize the CHP-produced thermal energy for cooling of PV-CHP system.[8] These trigen+PV systems have the potential to save even more energy.
The largest deployment of micro-CHP is in Japan at this time (2009), where over 90,000 units are in place, with the vast majority being of the "ECO-WILL" type.[9] Six Japanese energy companies launched the 300 W–1 kW PEMFC ENE FARM[10][11] product in 2009, with 3,000 installed units in 2008, a production target of 150,000 units for 2009–2010 and a target of 2,500,000 units in 2030.[12]
It is estimated that about 1,000 micro-CHP systems were in operation in the UK as of 2002. These are primarily "Whispergen" Stirling engines, and Senertec Dachs reciprocating engines. The market is supported by the government through regulatory work, and some government research money expended through the Energy Saving Trust and Carbon Trust, which are public bodies supporting energy efficiency in the UK.[13] Effective as of 7 April 2005, the UK government has cut the VAT from 17.5% to 5% for micro-CHP systems, in order to support demand for this emerging technology at the expense of existing, less environmentally friendly technology. The reduction in VAT is effectively a 10.63%[14] subsidy for micro-CHP units over conventional systems, which will help micro-CHP units become more cost competitive, and ultimately drive micro-CHP sales in the UK. Of the 24 million households in the UK, as many as 14 to 18 million are thought to be suitable for micro-CHP units. A factory in Horsham UK for the production of SOFC based micro-CHP units is expected to start low-volume production in the second half of 2009[15]
In Germany, 3,000 ecopower micro-CHP units have been installed, using the U.S. based Marathon Engine Systems long-life engine. The engine runs on natural gas and propane. The ecopower micro-CHP is also available in the United States. A factory in Heinsberg, Germany for the production of SOFC based micro-CHP units started in June 2009 to produce 10,000 two-kilowatt units per year.[16] The German government is offering large CHP incentives, including feed-in tariffs and bonus payments for use of micro-CHP generated electricity. The United States federal government is offering a 10% tax credit for smaller CHP and micro-CHP commercial applications.
In South Korea subsidies will start at 80 percent of the cost of a domestic fuel cell.[17]
In 2007, the United States company "Climate Energy" of Massachusetts has introduced its flagship product named "Freewatt"[4], a micro-CHP system for sale in the Northeast US domestic and small business market. The Freewatt system uses a Honda MCHP engine bundled with a high-efficiency gas furnace (for warm air systems) or boiler (for hydronic or forced hot water heating systems). The generator alone is not available for purchase separate from the furnace. The engine generates up to 1.2 kW of electricity at 20% efficiency. Most of the waste heat is captured for space heating, resulting in 85% efficiency overall. The system generates electricity opportunistically, only when space heating is needed during cold weather. The system is priced at about $14k, with the furnace being worth about $5k.
The engine generator has been demonstrated to run about 4000 hours per year (nearly 50% duty), and produce up to 5000 kWh of electricity per year, worth about $1k at the residential level. The system is internet connected and after about 4000 hours, it automatically sends an email requesting service. The main purpose of regular service is for an engine oil change.
It has been demonstrated in dozens of homes that Freewatt will produce about 50% of the electric power needs of a typical US home from the fuel now used to heat the home (which must be natural gas or propane), thereby doubling the value of the heating fuel purchased by the homeowner and significantly reducing the carbon footprint of the home by reducing electricity demand and emissions from large centralized power plants.
The product has already received numerous awards, including breakthrough product of the year from Popular Mechanics Magazine, and is expected by some to be widely available throughout the United States in 2008 or 2009. Estimates are that this product could be used in about 50 million homes in the United States. The Freewatt product is being brought to the national US and Canadian market by ECR International [5], the leading business partner of Climate Energy. ECR International is an 80 year old company with extensive line of heating and cooling products for the home.
Through a pilot program scheduled for mid-2009 in Southern Ontario, the Freewatt system is being offered exclusively by Eden Oak and offers homeowners a new option for controlling their energy costs and reducing their environmental footprint. The initiative, which reflects the collaborative efforts of the Government of Canada, Eden Oak, ECR International, Enbridge Gas Distribution and National Grid, will involve the sale and installation of the Freewatt systems in new Eden Oak communities in the Toronto area.
Trenergi Corp., Hopkinton, Massachusetts, an early stage company, in June 2010, announced its Trion residential, high-temperature (300 F) micro-CHP, in proof-of-concept stage, that operates on both gas and oil, demonstrating combined heat and electrical power efficiencies of up to 90%. Their first products will be 1, 3, and 5 kW units.
The advantage of having some "ownership" of one's electrical power was discussed above. Actual utility bill savings are probably minimal when looking at life-cycle cost of this approach as compared to a simple natural gas furnace.
There are definite pollution-reduction advantages if the unit is replacing an electric heating system powered by a coal power plant.
Also, like other distributed power systems, the end user can configure the unit as an emergency power source in the event of a power outage.
A big picture advantage of this approach is the ability to distribute power generation, locally, at the end-user rather than a remote power plant. If deployed on a large scale, this can reduce the need for new power plant installations and free-up transmission line capacity for other uses (e.g. solar energy or wind turbine farms). There is also the reduced long-range transmission losses. Avoiding transmission line losses and power plant construction reduces costs, energy consumption and pollution for everyone. In a fully realized distributed power generation scenario, micro-CHP may be used to supplement the less predictable generation provided by solar, wind and other energy sources, and therefore increase grid reliability.
Testing is underway in Ameland, the Netherlands for a three year field testing until 2010 of HCNG were 20% hydrogen is added to the local CNG distribution net, the appliances involved are kitchen stoves condensing boilers and micro-CHP boilers.[18][19]
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