Micro combined heat and power

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Micro combined heat and power or microCHP is an extension of the now well established idea of cogeneration to the single/multi family home or small office building.

Overview

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 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 energy crisis of the 1970s. 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.

Micro-CHP systems

Micro-CHP systems’ chief difference from their larger-scale kin is in the operating parameter-driven operation. In many cases industrial CHP systems primarily generate electricity and heat is a useful by-product. Contrarily, micro-CHP systems, which operate in homes or small commercial buildings, are driven by heat-demand, delivering electricity as the byproduct. Because of this operating model and because of the fluctuating electrical demand of the structures they would tend to operate-in, homes and small commercial buildings, 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]

Engine Types and Technologies

Micro-CHP systems are currently based on several different technologies:

• Internal combustion engines
• Stirling engines
• Steam engines
• Microturbines

Fuels and Engine Types

The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly (though it does emit CO₂), 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, because it burns without producing ash, soot or tar. 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.

Fuels

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.

Engines

External combustion engines, can run on any high-temperature heat source. These engines include the Stirling engine, and the steam engine. Both range from 10%-20% efficient, and as of 2008, small quantities are in production for micro-CHP products. Other possible heat cycles include the Organic Rankine Cycle (lower heat),Ericsson cycle, and Stoddard cycle.

Fuel cel Mchp

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. The advantages over sterling CHP are no moving parts, less maintenance, and quieter operation. The surplus electricity can be delivered back to the grid^[2].

Targets

DOE Technical Targets: 1–10 kWe residential combined heat and power fuel cells operating on natural gas^[3].

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.

Market status and government policy

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^[4]. Six Japanese energy companies launched the 300W-1kW PEMFC ENE FARM^[5] 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^[6].

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^[7]. 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%^[8] 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. [1] 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^[9].

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 2 kW units per year^[10]. 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^[11].

Advantages

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 (i.e. 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.

Disadvantages

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Some of the concerns of this technology are that gas fuels have historically had high price volatility. This makes it difficult to forecast the system's operating cost. Also the fuels are non-renewable with a net release of carbon dioxide. Gas leaks are very hazardous. Units operating on vegetable oil, however, (as has been done in Germany with approximately 1800 installed units as of 2007), use a renewable and very safe fuel.^{[citation needed]} Some vendors will not sell the generator alone, but only packaged with a new gas furnace. Despite what the salesmen say, there is no legitimate reason for this, because a generator and a furnace can run off separate thermostats, thus avoiding issues of compatibility and liability. The structuring of the products in this way may be a concession made in deals brokered with gas companies for rebates.^{[citation needed]}

A practical disadvantage is the "energy balance" of the generator. Often in the summer months for example, electricity is required but only heat for hot water, meaning that either the power production must be limited dynamically by the heat demand, so that residual power demand must be satisfied from the grid, or some of the heat produced must be cooled away and means has to be provided to achieve this. In practice there are few occasions when the equipment produce exactly the required local mix of heat and electricity, so cooling and/or power exchange with other systems are necessary. In practice all this means that not all of the local energy requirements can be met by the CHP equipment alone unless there is to be energy (heat) dumping. Large CHP plants can turn surplus heat in the summer months into cooling by means of complex and expensive absorption refrigeration plants, but this is generally not economical on a small scale.^{[citation needed]}

Another issue is the low electrical efficiency of very small micro CHP in the size range around 1 kWe. A report by the UK Carbon Trust suggested that at this scale there may be no CO2 saving from micro-CHP, compared to using an efficient condensing boiler for one's heat and generating one's electricity from a gas CCGT power station. (By contrast, large CHP plant does make a sizeable saving compared to separate production of heat and electricity).^{[citation needed]}

Large premises are likely to be the most economical advantageous for micro-CHP, especially where heat demand is relatively stable throughout the year. (For example in apartment buildings, hospitals or nursing homes where there are significant need for hot water for bathing).^{[citation needed]}

It follows from the above that in sizing CHP equipment to make the most economical energy saving, the key factors are the minimum heat load required and any possibilities of storing surplus heat. If this is zero for any appreciable time, then the investment in a micro-CHP system is less financially viable. A possible exception to this might be in off-the-grid locations where mains electricity is not available and local generation of power is the only option.^{[citation needed]}

Research

Testing is underway in Ameland for a 3 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^{[12] [13]}.

See also

Energy portal

• Distributed generation
• District heating
• Geothermal power in Iceland
• Grid-tied electrical system
• Home fuel cell
• Microgeneration Certification Scheme
• Pinch analysis
• Relative cost of electricity generated by different sources
• Stationary fuel cell applications
• Trigeneration
• Virtual power plant

References

Codes and standards

• Fuel cell gas appliances up to 70 kW
• Installation permitting guidance for hydrogen and fuel cells stationary applications
• Standard for the installation of stationary fuel cell power systems

External links

• DOE fuel cells used in combined heat and power
• EU-HEGEL a reciprocating engine CHP
• Information about the Warm Air Freewatt Cogeneration System
• Mini and micro CHP – Market Assessment and Development Plan
• Micro CHP project in the Netherlands
• Accelerating the Development of Micro and Mini Cogeneration
• CSMonitor: It heats. It powers. Is it the future of home energy?
• Micro CHP information, products, economics, publications
• the World Alliance for Decentralized Energy
• Information about the Disenco HomePowerPlant