Heat pumps

Like district heating and combined heat and power, heat pumps are already widely used in continental Europe, but strangely rare in Britain. Heat pumps are back-to-front refrigerators. Feel the back of your refrigerator: it's warm. A refrigerator moves heat from one place (its inside) to another (its back panel). So one way to heat a building is to turn a refrigerator inside-out - put the inside of the refrigerator in the garden, thus cooling the garden down; and leave the back panel of the refrigerator in your kitchen, thus warming the house up. What isn't obvious about this whacky idea is that it is a really efficient way to warm your house. For every kilowatt of power drawn from the electricity grid, the back-to-front refrigerator can pump three kilowatts of heat from the garden, so that a total of four kilowatts of heat gets into your house. So heat pumps are roughly four times as efficient as a standard electrical bar-fire. Whereas the bar-fire's efficiency is 100%, the heat pump's is 400%. The efficiency of a heat pump is usually called its coefficient of performance or CoP. If the efficiency is 400%, the coefficient of performance is 4.

Heat pumps can be configured in various ways (figure 21.10). A heat pump can cool down the air in your garden using a heat-exchanger (typically a 1-metre tall white box, figure 21.11), in which case it's called an air-source heat pump. Alternatively, the pump may cool down the ground using big loops of underground plumbing (many tens of metres long), in which case it's called a ground-source heat pump. Heat can also be pumped from rivers and lakes.

Some heat pumps can pump heat in either direction. When an air-source heat pump runs in reverse, it uses electricity to warm up the outside air and cool down the air inside your building. This is called air-conditioning. Many air-conditioners are indeed heat-pumps working in precisely this way. Ground-source heat pumps can also work as air-conditioners. So a single piece of hardware can be used to provide winter heating and summer cooling.

People sometimes say that ground-source heat pumps use "geother-mal energy," but that's not the right name. As we saw in Chapter 16, geothermal energy offers only a tiny trickle of power per unit area (about 50mW/m2), in most parts of the world; heat pumps have nothing to do with this trickle, and they can be used both for heating and for cooling. Heat pumps simply use the ground as a place to suck heat from, or to dump heat into. When they steadily suck heat, that heat is actually being replenished by warmth from the sun.

There's two things left to do in this chapter. We need to compare heat pumps with combined heat and power. Then we need to discuss what are the limits to ground-source heat pumps.

Figure 21.11. The inner and outer bits of an air-source heat pump that has a coefficient of performance of 4. The inner bit is accompanied by a ball-point pen, for scale. One of these Fujitsu units can deliver 3.6 kW of heating when using just 0.845 kW of electricity. It can also run in reverse, delivering 2.6 kW of cooling when using 0.655 kW of electricity.

Figure 21.11. The inner and outer bits of an air-source heat pump that has a coefficient of performance of 4. The inner bit is accompanied by a ball-point pen, for scale. One of these Fujitsu units can deliver 3.6 kW of heating when using just 0.845 kW of electricity. It can also run in reverse, delivering 2.6 kW of cooling when using 0.655 kW of electricity.

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Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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