Electricity “Foregone”- Maximising CHP’s Virtual COP

In the Introduction to the government’s Heat Strategy (The Future of Heating), the chief Scientific Advisor to DECC, Professor MacKay, makes the point that waste heat from power generation isn’t free. When you make use of the waste heat for space heating or industrial processes, it comes at a cost – the cost being the loss of electricity generation. He calls it the electricity “foregone”. He then goes on to compare CHP to electric heat pumps. He says that CHP systems can be regarded as “virtual” heat pumps.  Both technologies effectively require some power to produce heat. What you might know as a CHP system’s Z factor is the same as the heat pumps Co-efficient of Performance (COP) – the ratio of power in (or foregone) to heat out.

Professor McKay goes on to state that whether we use heat pumps or CHP what we need to ensure that this COP (real or virtual) is as high as possible i.e. the minimum power use for the maximum heat delivered and the key to this is to reduce heating system temperatures to as low as possible. And to do this we need to insulate homes – that way our current radiators will still be able to adequately heat our homes even though we’ve reduced the temperatures at which they operate (in order to maximise our CHP/heat pump COP).

But temperatures aren’t the only critical factor in maximising a CHP’s virtual COP. In my last blog I talked about the idea of nuclear power operating in CHP mode. I stated that the COP for a nuclear power station could be as high as 14 (in fact the graph below shows this as just over 16) – based on 2 key design principles.  The first is to do with design and operation of the district heating system itself to achieve low flow and return temperatures. The second key principle, that no-one seems to be discussing or be aware of, is the design of the power station itself – or more specifically the power station’s steam turbine. This design idea has been put forward by William Orchard and while his other ideas about designing for optimal temperatures seems to be gaining ground, this idea seems to be largely unknown.

Almost all large thermal power stations in the UK use a steam turbine to generate electricity. Sometimes (in the case of gas combined cycle CCGT power stations) they combine a steam turbine with a gas turbine which gives an overall higher efficiency of electrical generation. The hot exhaust gases from the gas turbine are used to raise steam for the steam turbine. But you need your fuel in gaseous form to be able to do this – hence why this combining of cycles is generally limited to gas.  There are also power stations that use gas turbines on their own but these are predominantly used for peak load and operate for very few hours in the year.

So when we talk about large power stations operating in CHP mode it’s generally the heat that we extract from the steam turbine that is used for district heating. Conventionally when steam turbines operate in CHP mode there is just one point of extraction for the heat from the turbine. The innovation that William Orchard proposes consists of two parts. The first is to have multiple stages of extraction from the turbine and the second is to design the turbine so that the heat is extracted at lower temperatures and pressures using an automated system. By doing this you could dramatically reduce the electricity “foregone” when the heat is extracted. The following chart shows how this plays out for different types of power station and different flow and return temperatures. Three stages of extraction from a CCGT with a 75 degree flow and 25 degree return gives a COP of 17. For nuclear, at the same temperatures, the result is slightly lower at just over 16. In characteristic form William also shows how this compares to an air source heat pump with a COP of 3.

The implication of this is that if we do go ahead with building new nuclear power stations, we have a choice. We can build them with the turbines optimised for CHP and build district heating connected to them  in which case we only need to build an additional 6% of electrical capacity to make up for the electricity “foregone”. If we go down an electric heat pump route then (assuming a COP of 3) we’ll need an additional 33% of electrical capacity to supply the equivalent heat output.

In my next blog find out more out how these turbine modifications work and what happened when William wrote to turbine manufacturers to start making turbines in this way.

Source: ©Copyright Orchard Partners London Ltd 2012

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