Microcombined heat and power CHP

It is interesting how two nineteenth century technologies, the Stirling engine and the fuel cell, are only now coming into their own. Invented by Robert Stirling in 1816, the engine that bears his name is described as an 'external combustion engine'. This is because heat is applied to the outside of the unit to heat up a gas within a sealed cylinder. The heat source is at one end of the cylinder whilst the cooling takes place at the opposite end. The internal piston is driven by the successive heating and cooling of the gas. When the gas is heated it expands, pushing down the piston. In the process the gas is cooled and then pushed to the heated top of the cylinder by the returning piston, once again to expand and repeat the process. Because of advances in piston technology and in materials like ceramics from the space industry and high temperature steels allowing temperatures to rise to 1200°C, it is now considered a firm contender for the micro-heat-and-power market.

Heat can be drawn off the engine to provide space heating for a warm air or wet system. Alternatively it can supply domestic hot water. In one system on the market, 'Whispergen', the vertical motion of the piston is converted to circular motion to power a separate generator.

MicroGen is currently conducting trials of a Stirling CHP system in which the generator is contained within the cylinder. At the heart of the system is a unique technology developed by Sunpower in the US. This consists of a sealed chamber containing a single piston integrated with an alternator. At the top of the piston there is a magnate which interacts with the alternator coil to produce electricity at 240 volts single phase. (Figure 7.6)

The top of the chamber is heated to a temperature of 500°C whilst the lower part is water cooled to 45°C creating the necessary pressure difference in the captive gas. The MicroGen unit is designed to produce 1.1 kW of electricity which is considered to be adequate for base load for most domestic requirements. Any further load will be drawn from the grid in the normal way.

Because the system is strictly controlled to 240 volts and 50 Hz it is compatible with mains electricity and therefore it is claimed that it can be linked directly to the domestic ring main. It will be possible to feed

Expanding gas

Contracting gas

Displacer piston

Alternator power piston

Displacer piston

Alternator power piston

Heat in from gas burner

The displacer moves gas from the hot to the cold end of the chamber whether expanding or contracting

Heat in from gas burner

The displacer moves gas from the hot to the cold end of the chamber whether expanding or contracting

Expanding gas

Alternator generates electricity and also kick-starts the engine

Alternator generates electricity and also kick-starts the engine

Contracting gas

Heat out through water cooling

Water cooling coupled with heat creates a pressure wave

Heat out through water cooling

Water cooling coupled with heat creates a pressure wave

Planar spring keeps displacer moving up and down

Planar spring keeps displacer moving up and down

Figure 7.6

Four phases of the Stirling engine excess electricity to the grid. The four phases of the Stirling cycle are cycle explained in Figure 7.6.

As regards the warming function, the heat which is drawn off by the water coolant is reinforced by heat from the flue gases extracted by a heat exchanger. There is a supplementary heating element in the system for occasions when demand exceeds the output from the engine. The heat is transmitted either to a condensing or combi-boiler. There are three heat output options ranging from 15kW (51 000 btu/h) to 36 kW (122000 btu/h). All models will be able to reduce their heating output to 5 kW when necessary. It is expected that future models will be adapted to serve warm air heating systems. There is also the possibility of creating a multiple unit system to provide increased power and heat at a commercial scale.

Because there is only one moving part within the closed chamber the Stirling engine requires no maintenance. The boiler element needs the same level of maintenance as a conventional boiler.

The cost estimate is that the system will pay back the additional cost over and above a conventional boiler in 4-5 years.

The unit is compact, can fit between modular kitchen units and creates a noise level comparable to an average refrigerator (Figure 7.7).

Despite some current regulatory problems in the UK the Department of Environment, Transport and the Regions is optimistic about the prospects for micro-CHP or 'micro-cogeneration', estimating the potential domestic market to be up to 10 million units. With the opening up of the energy markets, micro-CHP is likely to become a major player in the energy stakes, accounting for some 25-30 GW of electricity (GWe). One of the factors favouring this technology is that it can be up to 90 per cent efficient and result in a reduction in total carbon dioxide emissions of up to 50 per cent when compared with the separate production of heat and energy. Large power stations are about 30 per cent

efficient. Add to this line losses of 5-7 per cent and it is obvious there is no contest.

In summary, the advantages of micro-CHP or micro-cogeneration are:

• It is a robust technology with few moving parts.

• Maintenance is simple, consisting of little more than cleaning the evaporator every 2000-3000 hours (on average once a year).

• Since there is no explosive combustion the engine produces a noise level equivalent to a refrigerator.

• It is compact with a domestic unit being no larger than an average refrigerator.

• It operates on natural gas, diesel or domestic fuel oil. In the not distant future machines will be fuelled by biogas from the anaerobic digestion of waste.

• The efficiency is up to 90 per cent compared with 60 per cent for a standard non-condensing boiler.

• Unlike a boiler it produces both heat and electricity, reducing energy use by about 20 per cent and saving perhaps £200-£300 on the average annual electricity bill.

• It can be adapted to provide cooling as well as heat.

The UK government is keen to promote this technology and it is always worth checking if grants are available. The best source of advice is the

Energy Saving Trust (www.est.org.uk).

Solar Stirling Engine Basics Explained

Solar Stirling Engine Basics Explained

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.

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