Micro combined heat and power, micro-CHP, μCHP or mCHP is an extension of the idea of cogeneration to the single/multi family home or small office building in the range of up to 50 Kilowatt. Usual technologies for the production of heat and power in one common process are e.g. internal combustion engines, Microturbine, or .
Local generation has the potential for a higher efficiency than traditional grid-level generators since it lacks the 8-10% energy losses from transporting electricity over long distances. It also lacks the 10–15% energy losses from District heating in heating networks due to the difference between the thermal energy carrier (hot water) and the colder external environment.
The most common systems use natural gas as their primary energy source and emit carbon dioxide; nevertheless the effective efficiency of CHP heat production is much higher than of a condensing boiler, and thus reducing emissions and fuel costs.
A micro-CHP generator may primarily follow heat demand, delivering electricity as the by-product, or may follow electrical demand to generate electricity, with heat as the by-product. When used primarily for heating, micro-CHP systems may generate more electricity than is instantaneously being demanded; the surplus is then fed into the grid.
The purpose of cogeneration is to make use of more of the chemical energy in the fuel. The reason for using CHP systems is that large thermal power plants which generate electric power by burning fuel produce between 40% and 60% low-temperature waste heat, due to Carnot's theorem.
Micro CHP systems allow highly efficient cogeneration while using the waste heat even if the served heat load is rather low. This allows cogeneration to be used outside population centers, or even if there is no district heating network. It is efficient to generate the electricity near the place where the heat can also be used. Small power plants (μCHP) are located in individual buildings, where the heat can be used to support the heating system and recharge the hot domestic water tank, thus saving heating oil or heating gas. CHP systems are able to increase the total energy utilization of primary energy sources. Thus CHP has been steadily gaining popularity in all sectors of the energy economy, due to the increased costs of electricity and fuel, particularly fossil fuels, and due to environmental concerns, particularly climate change.
In a traditional power plant delivering electricity to consumers, about 34.4% of the primary energy of the input fuel, such as coal, natural gas, nuclear power, petroleum, solar thermal, or biomass, reaches the consumer via electricity, 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 water heating or space heating. In total, over 90% of the heat from the primary energy source (LHV based) can be used when heat production does not exceed the thermal demand.
Since 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 cost-effectiveness of , , steam engines, gas turbines, and .
Combined heat and power (CHP) systems for homes or small commercial buildings are usually fueled by natural gas to produce electricity and heat. If no access to the natural gas network is available, which in general is the cheapest alternative, LPG, LNG or heating fuel (diesel) might be an alternative. The PEMFC fuel cell mCHP operates at low temperatures (50 to 100 °C) and needs high purity hydrogen. It is prone to contamination; changes are made to operate at higher temperatures and improvements on the fuel reformer. The SOFC fuel cell mCHP operates at a high temperature (500 to 1,000 °C) and can handle different fuel sources well, but the high temperature requires expensive materials to handle it; changes are made to operate at a lower temperature. Because of the higher temperature the SOFC in general has a longer start-up time and needs continuous heat output even at times when there is no thermal demand.
CHP systems linked to absorption chillers can use waste heat for refrigeration. Tri-Generation success story
A 2013 UK report from Ecuity Consulting stated that MCHP is the most cost-effective method of utilizing gas to generate energy at the domestic level. The role of micro CHP in a smart energy world Micro CHP report powers heated discussion about UK energy future
The fuel cell industry review stated in 2013 that with 64% of global sales the fuel cell micro-combined heat and power had passed the conventional engine-based micro-CHP systems in sales in 2012. The fuel cell industry review 2013
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 over 60 million 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 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.
External combustion engines can run on any high-temperature heat source. These engines include the Stirling engine, hot "gas" turbocharger, and the steam engine. Both range from 10%-20% efficiency, and as of 2014, 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 anyway for purposes of space heating.
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.
PEMFC fuel cells fueled by natural gas or propane use a steam reformer to convert methane in the gas supply into carbon dioxide and hydrogen; the hydrogen then reacts with oxygen in the fuel cell to produce electricity. A PEMFC fuel cell based micro-CHP has an electrical efficiency of 37% LHV and 33% HHV and a Energy 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. An estimated 138,000 Fuel cell CHP systems below 1 kW had been installed in Japan by the end of 2014. Most of these CHP systems are PEMFC based (85%) and the remaining are SOFC systems.
In 2013 Service life is around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years. Latest developments in the Ene-Farm scheme
United States Department of Energy (DOE) Technical Targets: 1–10 kW residential combined heat and power fuel cells operating on natural gas. DOE Distributed/Stationary fuel cell systems
| +Development of fuel cells |
| 45% |
| 90% |
| $450/kW |
| 2 min |
| 20 min |
| 0.3%/1000 h |
| 60,000 h |
| 99% |
CPVT systems are currently in production in Europe, Renewable Energy World-Researchers explore hybrid concentrated solar energy system with Zenith Solar developing CPVT systems with a claimed efficiency of 72%.
Sopogy produces a micro concentrated solar power (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.
Another advantage of net-metering is that it is fairly easy to configure. The user's electrical meter can easily record electrical energy exiting as well as entering the home or business. 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 the 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 does have disadvantages: 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 2007, the United States company "Climate Energy" of Massachusetts introduced the "Freewatt, ANnalysis of data collected for the Freewatt microCHP system in Syracuse, NY a micro-CHP system based on a Honda MCHP engine bundled with a gas furnace (for warm air systems) or boiler (for hydronic or forced hot water heating systems).
Marathon Engine Systems, a Wisconsin company, produces a variable electrical and thermal output micro-CHP system called the ecopower with an electrical output of 2.2-4.7 kWe. The ecopower was independently measured to operate at 24.4% and 70.1% electrical and waste heat recovery efficiency, respectively.
Through a pilot program scheduled for mid-2009 in the Canadian province of Ontario, the Freewatt system is being offered by home builder Eden Oak with support from ECR International, Enbridge Gas Distribution and Electrical grid.
Micro-CHP Accelerator, a field trial performed between 2005 and 2008, studied the performance of 87 Stirling engine and internal combustion engine devices in residential houses in the UK. This study found that the devices resulted in average carbon savings of 9% for houses with heat demand over 54 GJ/year.
An ASME (American Society of Mechanical Engineers) paper fully describes the performance and operating experience with two residential sized Combined Heat and Power units which were in operation from 1979 through 1995.Frederick R. Rosse: EXPERIENCE WITH EARLY DISTRIBUTED GENERATION SYSTEMS , Proceedings of IJPC-2003 2003 International Joint Power Conference, paper IJPGC2003-40192
Oregon State University, funded by the U.S. Department of Energy's Advanced Research Project Agency - Energy (ARPA-e), tested the state of the art micro-CHP systems in the United States. The results showed that the nominally 1 kWe state-of-the-art micro-CHP system operated at an electrical and total efficiency (LHV based) of 23.4 and 74.4%, respectively. The nominally 5 kWe state-of-the-art system operated at an electrical and total efficiency (LHV based) of 24.4 and 94.5%, respectively. The most popular 7 kWe home backup generator (not CHP) operated at an electrical efficiency (LHV based) of 21.5%. The price of the emergency backup generator was an order of magnitude lower than the 5 kWe generator, but the projected life span of the system was over 2 orders of magnitude lower. These results show the trade-off between efficiency, cost, and durability.
The U.S. Department of Energy's Advanced Research Project Agency - Energy (ARPA-e) has funded $25 million towards mCHP research in the GENerators for Small Electrical and Thermal Systems (GENSETS) program. 12 project teams have been selected to develop a 1 kWe mCHP technology that can achieve 40% electrical efficiency, have a 10-year system life, and cost under $3000.
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