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.

Contents

[edit] 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 usually not considered 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 impractical to transport waste heat, an energy efficient system must generate electricity only at locations where the waste heat can be put to good use. In a central power plant, the supply of "waste heat" often exceeds the demand, so the waste heat has very little economic value. Then the waste heat is typically dissipated in a cooling tower without ever being used purposefully. One way to make better use of the "waste heat", is to consume the primary energy source on-site, and thus generate energy in all of the required forms, at the point-of-use. This is known as a combined heat and power (CHP) system, or "cogeneration".

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 50% for newer plants. In contrast, a CHP system typically converts 10%-20% of the primary heat to electricity, and most of the remaining heat is captured for hot water or space heating. Typically about 10%-30% of the heat is dissipated without being used. In total, at least 65% and often as much as 90% of the heat from the primary energy source goes to useful purposes.

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 Stirling engines, steam engines, gas turbines, diesel engines and Otto engines.

[edit] 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 net-metering is 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 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.

[edit] System Types and Technologies

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

[edit] Fuels and Engine Types

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, 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.

[edit] 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, 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), and natural gas.

[edit] 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 Ericsson cycle, and Stoddard cycle.

[edit] Market status and government policy

The largest deployment of micro-CHP is in Japan at this time, where over 50,000 units are in place, with the vast majority being of the "ECO-WILL" type based on Honda's MCHP engine generator unit. It is estimated that about 1,000 micro-CHP systems are 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. 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 12.5% 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.

[edit] Honda MCHP unit

The Honda MCHP unit is a high-endurance engine with extremely low noise output (40 dB) and solid-state grid interface. It runs on natural gas or propane. These fossil fuels are less expensive than gasoline/petrol, but more expensive for example than renewable biomass sometimes used to power external combustion engines.

[edit] Products and distributors

Around 2007, the United States company "Climate Energy" of Massachusetts has introduced its flagship product named "Freewatt"[2], a micro-CHP system for sale in the US domestic and small business market. The Freewatt system uses a Honda MCHP engine bundled with a 100 kBTU gas furnace. The system is offered for $14k, with the furnace being worth about $5k. The generator alone is not available for purchase separate from the furnace. The base price offer includes 3 years of service, and 5 year parts and warranty. 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 engine generator is projected 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 is claimed that this system 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, although the market is limited because the furnace can not be purchased separately, and it must use natural gas or propane.

One major United States natural gas distribution company, KeySpan, has closely evaluated this product and is offering a substantial monetary incentive (a $2600 rebate) for its customers whom purchase this product and thereby help in its energy conservation program. Some of the concerns are over the historic price volatility of natural gas, the cost of the system and fuel, and the use of a non-renewable fuel with a net release of carbon dioxide.

The software and hardware that interfaces the furnace, thermostat and engine controller was developed in Massachusetts.

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