3.2.3 MICRO-CHP SYSTEMS

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, micro-CHP systems will often generate more electricity than is instantly being demanded. For this reason it is necessary that the CHP is connected to the distribution system.

To date, micro-CHP systems achieve much of their savings, and thus attractiveness to consumers, through a net metering model wherein home-generated power exceeding the instantaneous in-home needs is sold back to the electrical utility. 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.

The majority of cogeneration systems use natural gas for fuel, because natural gas burns easily and cleanly, it is available in most areas and is easily transported through pipelines. 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 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 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, woodgas, and natural gas, as well as multi-fuel systems.

Integration with home energy systems

In order to be viable in domestic installations it is essential that micro CHP is compatible with the operational parameters of central heating, such as water flow rates and temperatures and that it does not require the addition of, for example, large storage tanks to provide thermal buffering. It is also important to bear in mind that micro CHP does not respond well to rapid on-off cycling and that engines are normally designed to meet about 60% of the peak design load. This maximizes useful run hours under average winter conditions, and normally leads to the bulk of annual demands being met by the primary system.

However, some form of supplementary heating may be required in sever weather conditions and to achieve rapid heat up, for example, after the home has been unoccupied for some time.

Economic benefits and barriers

The economic viability of micro CHP depends on both the marginal capital investment (compared with a gas boiler) and the value of electricity produced by the unit. For any given system, therefore, the payback relies on the unit's operating hours and consequently the total kWh produced annually.

The table below illustrates the economics for a typical home with 18,000 kWh annual thermal demand. It can be seen that the value of the electricity is also dependent on whether it is consumed within the home or exported and sold to the energy supplier.

*The average electricity price in Bulgaria

Micro CHP fulfils the four key goals of the EU: security of supply, economic competitiveness, alleviation of fuel poverty and mitigation of climate change.

One of the most significant potential barriers to the micro CHP is the ability or otherwise to connect the system to the electricity supply network. Although it is possible to run the units in isolation (given appropriate energy storage and control systems) this would negate the economic benefits. Domestic electrical loads are extremely volatile with baseloads of around 100 W, average 400-600W and peak loads upwards of 15-20kW. The simplest solution is therefore to use the network as the balancing system with surplus generation exported and any shortfall imported as is normal practice.

Market status, Government Policy and Financial Supporting Scheme

After studying both the energy needs (electricity and heat energy) of a given family or a small enterprise, as well as the possibilities of constructing a small RES-e or/and micro-CHP, the market opportunities in the respective country are studied.

The EU Energy Policy, as well as the policies of the different member states support the maximal use of RES in the countries. In compliance with this support the Governments adopted different financial supporting schemes. In several countries the construction of small RES-e and micro-CHP installations is subsidized, and in others the produced electricity from RES and CHP is purchased from the producers obligatory at preferential prices.

The investor of a given small RES system of micro CHP has to get familiar with the existing financial supporting schemes in his/her country and to take advantage of them.