Heat pumps compared with combined heat and power

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I used to think that combined heat and power was a no-brainer. "Obviously, we should use the discarded heat from power stations to heat buildings rather than just chucking it up a cooling tower!" However, looking carefully at the numbers describing the performance of real CHP systems, I've come to the conclusion that there are better ways of providing electric ity and building-heating.

I'm going to build up a diagram in three steps. The diagram shows how much electrical energy or heat energy can be delivered from chemical energy. The horizontal axis shows the electrical efficiency and the vertical axis shows the heat efficiency.

The standard solution with no CHP

In the first step, we show simple power stations and heating systems that deliver pure electricity or pure heat.

Condensing boiler

(A)

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worse

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Electrical efficiency (%)

Electrical efficiency (%)

Condensing boilers (the top-left dot, A) are 90% efficient because 10% of the heat goes up the chimney. Britain's gas power stations (the bottom-right dot, B) are currently 49% efficient at turning the chemical energy of gas into electricity. If you want any mix of electricity and heat from natural gas, you can obtain it by burning appropriate quantities of gas in the electricity power station and in the boiler. Thus the new standard solution can deliver any electrical efficiency and heat efficiency on the line A-B by making the electricity and heat using two separate pieces of hardware.

To give historical perspective, the diagram also shows the old standard heating solution (an ordinary non-condensing boiler, with an efficiency of 79%) and the standard way of making electricity a few decades ago (a coal power station with an electrical efficiency of 37% or so).

Combined heat and power

Next we add combined heat and power systems to the diagram. These simultaneously deliver, from chemical energy, both electricity and heat.

Electrical efficiency (%)

Electrical efficiency (%)

Each of the filled dots shows actual average performances of CHP systems in the UK, grouped by type. The hollow dots marked "CT" show the performances of ideal CHP systems quoted by the Carbon Trust; the hollow dots marked "Nimbus" are from a manufacturer's product specifications. The dots marked "ct" are the performances quoted by the Carbon Trust for two real systems (at Freeman Hospital and Elizabeth House).

The main thing to notice in this diagram is that the electrical efficiencies of the CHP systems are significantly smaller than the 49% efficiency delivered by single-minded electricity-only gas power stations. So the heat is not a "free by-product." Increasing the heat production hurts the electricity production.

It's common practice to lump together the two numbers (the efficiency of electricity production and heat production) into a single "total efficiency" - for example, the back pressure steam turbines delivering 10% electricity and 66% heat would be called "76% efficient," but I think this is a misleading summary of performance. After all, by this measure, the 90%-efficient condensing boiler is "more efficient" than all the CHP systems! The fact is, electrical energy is more valuable than heat.

Many of the CHP points in this figure are superior to the "old standard way of doing things" (getting electricity from coal and heat from standard boilers). And the ideal CHP systems are slightly superior to the "new standard way of doing things" (getting electricity from gas and heat from condensing boilers). But we must bear in mind that this slight superiority comes with some drawbacks - a CHP system delivers heat only to the places it's connected to, whereas condensing boilers can be planted anywhere with a gas main; and compared to the standard way of doing things, CHP systems are not so flexible in the mix of electricity and heat they deliver; a CHP system will work best only when delivering a particular mix; this inflexibility leads to inefficiencies at times when, for example, excess heat is produced; in a typical house, much of the electricity demand comes in relatively brief spikes, bearing little relation to heating demand. A final problem with some micro-CHP systems is that when they have excess electricity to share, they may do a poor job of delivering power to the network.

Finally we add in heat pumps, which use electricity from the grid to pump ambient heat into buildings.

185%-efficient heat

185%-efficient heat

30%-efficient electricity, 80%-efficient heat oWartsila

Combined cycle gas Vb^

50 60

30%-efficient electricity, 80%-efficient heat

Back pressure steam 60

Pass out condensing steam turbinK ct

Gas turbine* CT Reciprocating engine ^

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Combined cycle gas Vb^

50 60

Electrical efficiency (%)

The steep green lines show the combinations of electricity and heat that you can obtain assuming that heat pumps have a coefficient of per-

formance of 3 or 4, assuming that the extra electricity for the heat pumps is generated by an average gas power station or by a top-of-the-line gas power station, and allowing for 8% loss in the national electricity network between the power station and the building where the heat pumps pump heat. The top-of-the-line gas power station's efficiency is 53%, assuming it's running optimally. (I imagine the Carbon Trust and Nimbus made a similar assumption when providing the numbers used in this diagram for CHP systems.) In the future, heat pumps will probably get even better than I assumed here. In Japan, thanks to strong legislation favouring efficiency improvements, heat pumps are now available with a coefficient of performance of 4.9.

Notice that heat pumps offer a system that can be "better than 100%-efficient." For example the "best gas" power station, feeding electricity to heat pumps can deliver a combination of 30%-efficient electricity and 80%-efficient heat, a "total efficiency" of 110%. No plain CHP system could ever match this performance.

Let me spell this out. Heat pumps are superior in efficiency to condensing boilers, even if the heat pumps are powered by electricity from a power station burning natural gas. If you want to heat lots of buildings using natural gas, you could install condensing boilers, which are "90% efficient," or you could send the same gas to a new gas power station making electricity and install electricity-powered heat pumps in all the buildings; the second solution's efficiency would be somewhere between 140% and 185%. It's not necessary to dig big holes in the garden and install under-floor heating to get the benefits of heat pumps; the best air-source heat pumps (which require just a small external box, like an air-conditioner's) can deliver hot water to normal radiators with a coefficient of performance above 3. The air-source heat pump in figure 21.11 (p147) directly delivers warm air to an office.

I thus conclude that combined heat and power, even though it sounds a good idea, is probably not the best way to heat buildings and make electricity using natural gas, assuming that air-source or ground-source heat pumps can be installed in the buildings. The heat-pump solution has further advantages that should be emphasized: heat pumps can be located in any buildings where there is an electricity supply; they can be driven by any electricity source, so they keep on working when the gas runs out or the gas price goes through the roof; and heat pumps are flexible: they can be turned on and off to suit the demand of the building occupants.

I emphasize that this critical comparison does not mean that CHP is always a bad idea. What I'm comparing here are methods for heating ordinary buildings, which requires only very low-grade heat. CHP can also be used to deliver higher-grade heat to industrial users (at 200 °C, for example). In such industrial settings, heat pumps are unlikely to compete so well because their coefficient of performance would be lower.

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