How Heat Pumps Compare To Combined Heat & Power | Clade Blog
One of the most important aspects of a successful transition to a zero-carbon economy will be the technology we use to heat and cool our buildings. In the UK our electricity grid has made great strides in cleaning up the way it produces power, but we lag behind in decarbonising heat. We’re still far too reliant on fossil fuels such as natural gas (mostly methane) and oil.
In previous blogs we have compared the efficiency, effectiveness and cost of heat pumps to that of gas boilers and hydrogen heating. In this blog we’re going to discuss how heat pumps fare compared to an approach called combined heat and power (CHP).
What is combined heat and power?
Combined heat and power, otherwise known as cogeneration, is defined as the generation of useful heat and electrical or mechanical power close to the point of use.
Combined heat and power is and has been available now for a long time, it can be deployed fairly easily and cost-effectively. Until the present it has been deployed mostly in large-scale industrial, commercial and public applications – several manufacturers have tried to bring domestic scale CHP to market but it’s never been very popular due to cost, complexity and a lack of government support. That’s why, as a technology, it’s not very widely discussed or understood.
There are many different types of CHP, the most common of which is the gas reciprocating engine. It is the most efficient type of system for 50/50 power and heat generation, which makes it most suitable for the common CHP use cases. Gas reciprocating engines are also easily available at competitive costs and easy to install and operate. Some larger power stations are considered to be CHP because they export heat through distinct heating but this is more common in the EU than the UK, so you’d be forgiven for not knowing about it. In the UK CHP is most often used at mid-scale – from 50kw to 5mw.
In existing applications CHP has operated at about 65-75% efficiency, which is an improvement over some other legacy technologies but does not compare that well to heat pumps, which can achieve up to 400% efficiency for heat production. Furthermore these existing systems often use fossil fuels and therefore contribute towards climate change.
What made CHP popular?
There are multiple commercial models which have made CHP an attractive investment vehicle. These are:
-On-site power generation at lower than grid cost. This power can either displace purchased power (which has been done, for example, in hospitals) or sold back to the grid (as it is in the case of large district heating).
-On-site heat generation that was, until fairly recently, rated low carbon in comparison to a high carbon grid and at roughly half the cost of a gas boiler. It was also an efficient way to generate heat as a by-product of generating power.
Why has the popularity of CHP now waned?
For power generation, the main change has been that that the UK grid has rapidly decarbonised to such an extent that in around 2018 CHP power became higher carbon than grid power. After this point it took a little while for the regulatory bodies to catch up and amend the calculations to reflect the change, but once they did the low-carbon CHP justification disappeared.
In terms of heat generation, the lower efficiencies of CHP make it too expensive compared to gas boilers. Without a supporting carbon reduction they simply cannot compete.
Is there a future for CHP?
Yes, potentially – CHP powered by waste or biomass means that some applications remain valid, for example where resilience of the power supply is necessary such as in hospitals. But for reciprocating gas engine CHP applications the low efficiency and high carbon output mean the technology is going the way of the dodo.
How combined heat and power compares to heat pumps
As a result of the progress the UK has made, heat pumps are now dramatically better for the environment than they were. For both residential and commercial use cases, heat pumps provide dramatic reductions in carbon emissions now and have negative zero impact on local air quality.
Combined heat and power, on the other hand, does not fare as well. Gas-fired CHPs create more NOx emissions than a standard boiler unless fitted with a catalytic converter. Even if they are fitted with a catalytic converter, they just about reach parity with a natural gas boiler – so still bad for the environment and air quality.
Biomass-fuelled combined heat and power is better – it’s classed as carbon neutral because any carbon released by burning the fuel is offset by the carbon absorbed by the trees grown to create the biomass fuel. Clearly, though, there are practical limitations on the widespread adoption of biomass CHP. These are: the availability of the biomass fuel (more difficult in urban environments – transporting the fuel over long distances will have an emissions impact) and the demand for heat (as mentioned before, without consistently high demand for heat the CHP will not operate at optimal efficiencies). Biomass CHP also requires significant emissions treatment to remove high levels of particulates and NOx, which can be expensive.
We would therefore not argue that heat pumps are always more desirable than biomass CHP. Rather, biomass CHP is only viable in certain use-cases. In businesses where there is a high demand for heat, and ready availability of biomass fuel, a biomass CHP is a great option for generating heat and power.
For most situations, however, the optimal solution is a heat pump which draws power from a low carbon grid for the following reasons:
-A heat pump can generate zero carbon heat because zero carbon power can now be purchased.
-A heat pump is comparatively low cost – it’s an all electric utility, and on a flex tariff with rewards for grid services it can ensure power is bought very cheaply.
-It is high efficiency – heat pumps can generate up to 400% efficiency.