CHP Webinar Follow Up Questions and Answers

CHP Webinar Q&A

Following our recent webinar: CHP Operation – Financial and Carbon Savings, we received a number of CHP questions from those that attended the webinar. We didn’t have time to answer them all during the webinar so our experts sat down to answer them all below.

If you missed the webinar, follow the link below to catch-up with everything that was discussed:

CHP Operation – Financial and Carbon Savings – Webinar

If you have any other questions that you would like to ask, please feel free to get in touch and we will do our best to answer them for you.

Question One

What are the best practices of CHPs across the UK used for district heating?


Chris Marsland: There are many sites currently utilising CHPs in DH schemes across the UK and I’m aware of at least one current tender for a new c.3.5MWe scheme. CHPs are good for providing the base heat load provided that there is a way of “selling” the electricity generated. This could be via a power purchase agreement (PPA) or direct to the tenants of the scheme through a private wire arrangement. CHPs in DH schemes are ideally suited for PPAs as they tend to have well-defined run hours that do not vary much which allows them to secure high-value contracts.

CHPs and their economics are also well understood by finance houses meaning that DH schemes utilising CHPs can be easier to get financed than some other technologies

Question Two

Andrew, .49kg/kWh electricity is one of the issues of this technology at the moment, what is the next stage of this technology?


Andrew Gardner: The figure of 0.49 is from 2014/2015 so you would need to use a more up to date figure to get a truer view of the emissions. CM will need to answer the second part.

Question Three

Is biomethane a good way to boost this technology? I mean if the biomethane comes from the grid.


Andrew Gardner: CHP technologies can use different gaseous fuels, so the introduction of bio-methane to the grid should not be a problem if the gas meets certain standards.

Chris Marsland: Biomethane injected into the grid can be a way of offsetting the carbon generated from a CHP. This can be simply through the general reduction of the amount of methane being burnt (a similar argument applies for the introduction of Hydrogen into the natural gas grid although that does present some challenges for the natural gas reciprocating engines) or through the direct purchase of the biomethane from the supplier (assuming the provenance of the fuel can be demonstrated and the consumption completely accounted for). There is an argument that if you have large quantities of bio-methane then injection into the gas grid is a better economic bet, but I have not seen any modelling to support this.

Question Four

Can you give guidance on annual maintenance costs? I noticed you allowed 0.03£/kW electricity generated?


Andrew Gardner: This is not something that can be easily explained and is normally calculated by the CHP maintainer, but we would normally expect the price to consist of three things:

  • Maintenance Cost
  • Deprecation of the Asset
  • Profit

Chris Marsland: Maintenance costs vary tremendously based upon the engine type, the length of contract and the level of cover provided by the contract. It is always best to ask the potential supplier for guidance. Maintenance rates of smaller CHPs tend to be quoted as pence per run hour rather than pence per kWh.

Question Five

The client will still have to pay electric mains incomer supply standing charges and the standing charges for the gas mains incomer supply will increase and presumably would need to be taken into account?


Andrew Gardner: Yes, these are normally factored into the costs/savings of the CHP

Chris Marsland: Correct, this is yet another variable that needs to be incorporated into the assessment of whether the CHP is right for a particular site.

Question Six

Does the lowering carbon content of grid electricity mean the end of the road for CHP?


Andrew Gardner: By no means, carbon savings only play a part in the viability of a CHP unit. Although the CO2 savings may be reduced, they will never be zero, as the heat savings are always there.

Chris Marsland: As with many things there are different ways to measure the carbon content of the grid electricity at any one time and then further ways as to how to interpret these figures. I can think of at least 4 current values of grid carbon that are in circulation and approved! I tend to use the figures from HM Treasury in their Green Book, this is not because I particularly think they are more accurate but as a source, HM Treasury are hard to argue with.

Whilst it is true that fossil fuel generation is rightly being replaced by low or zero-carbon renewables the full picture of this is not always apparent from the single number. The ADE recently undertook a study in which they demonstrated that natural gas CHP tended to displace marginal higher carbon generation, not renewable generation from the grid.

The future of CHP is linked to the final solution to the decarbonisation of heat. If we end up replacing the natural gas in the national gas grid with Hydrogen then CHP technology can continue in very much the same way as it does now, albeit with modified reciprocating engines to cope with the differing combustion properties of Hydrogen. If instead, we opt for the “all-electric future” then things are a little less certain although the massive increase in electrical demand and I suspect the needs to control the frequency of the grid will still leave opportunities for CHP although these may result in the CHPs running in a much more dynamic and interactive fashion, less than the typical 17 hours a day that most CHPs operate today.

Of course, what will probably emerge is a future where both solutions are implemented depending upon the final energy requirements of a site/consumer. This can only be good for CHP.

Question Seven

There are some DNOs that will oppose a CHP in certain areas, why is this and can it be overcome?


Chris Marsland: DNOs are facing an unprecedented strain on their networks due to the rapid rise in the amount of renewable generation being installed on their networks. There is little we can do about this except to work closely with the DNOs as they try to find solutions to these problems. All DNOs have their own types of solutions to these problems and often, with a little remodelling and small changes to a scheme connection can be accommodated. These challenges are here to stay and OFGEM is challenging the DNOs to improve how they handle these situations. The DNOs have a massive project called the Open Networks Project which will revolutionise how generation is connected to the network (amongst other things). This is always worth keeping an eye on for a signal to what the future may look like.

Question Eight

Why do you not need a BMS?


Chris Marsland: I was specifically referring to the series connection of a CHP into the LTHW heating system. The beauty of a CHP connected in a series arrangement is that it always sees the coldest return water from the heating system, with no intervention from control values of a boiler sequencer and its own, inbuilt, systems i.e. return temperature cut off and time clock are adequate to ensure correct operation.

Question 10

Are there some instances where electrically led CHP units are more efficient? 


Chris Marsland: There are circumstances where an electrically led CHP can be more economically viable, Last year a saw one industrial site where 2MWe of generation had a simple payback of 12 months. I have never seen an electrically led scheme be more efficient in its conversion of input energy to output energy than a heat led scheme.

Question Eleven

Would you recommend installing CHP units which modulate based on the site’s electrical load, and if so, in what kind of buildings?


Chris Marsland: Yes, if the heat load is present for such an operation. The need to modulate on electrical output could be brought about by a very high thermal to electrical load profile or as a condition imposed by the DNO for connection to the electricity network at that particular location (as per ENA G100). There is no particular kind of building where this is more likely however it does tend to work better where the site load is made up of many discrete loads rather than one or two large loads. The idea being that if one of say only two large loads switches off unexpectedly the CHP may have difficulty reducing its output in time to comply with the DNOs requirements whereas if there any many small loads constantly switching on and off the instantaneous load steps ten to be smaller and smoother allowing the CHP system to respond.

Question Twelve

Could data centres benefit from a CHP, even if it is electricity led? 


Andrew Gardner: Yes, Tri-generation could be a solution.

Chris Marsland: Data Centres have been on the CHP manufacturer/suppliers’ radar for many years however I am not aware of any large centres that have installed CHP. Theoretically data centres should work as the excess heat can be channelled into absorption chilling to help cool the equipment and the CHP can act as islanded generators in the case of a power outage.

I do not know exactly why CHPs have not taken off; I suspect several things may be influencing the choice but have no data/facts to confirm these.

Question Thirteen

What is the typical life of a CHP? How does the maintenance cost vary over the life of a CHP? 


Andrew Gardner: Typical life is 15 years; costs are dependent upon age and operation of the unit.

Chris Marsland: CHPs as with many regularly serviced items of mechanical equipment are like “Trigger’s Broom” i.e. after 15 years almost all of the parts will have been replaced and the unit will still run well. That said there are always opportunities to upgrade or improve the units. A good rule of thumb is to expect the smaller units (<500kWe) to last 10-15 years before they need to be replaced. Larger units can last much longer but can sometimes require removal from site for a major factory-based overhaul every 80000h or so.

See Q4 re maintenance but the maintenance costs of a CHP are quite spikey over time, just like a motor car, which is why I would always recommend a long term maintenance contract that enables you to “smooth out” the spikes over the length of the contract.

Question Fourteen

If we are heading towards a net-zero future, how far into the future do you see the benefits for CHP to be around? 


Andrew Gardner: There will always be a benefit to operating CHP as local generation, it is still the most efficient way to electricity and heat together.

Question Fifteen

  1. Is CHP eligible for RHI grants? 


Andrew Gardner: Yes, the CHPQA website as specific guidance on this.

Question Sixteen

Is there a way to calculate the maintenance costs of a system that has yet to be installed? Is there a rule of thumb? 


Andrew Gardner: Yes, this is covered by the CHP supplier when they carry out their site survey. A rule of thumb would be that the CHP electricity cost should be less than the site electricity cost with the gas cost removed.

Question Seventeen

Can CHP be used in any region?


Andrew Gardner: As long as there is a fuel supply, normally natural gas.

Chris Marsland: Yes, there are no geographic limitations provided an electrical connection is possible. Ideally there would be a connection to the national gas grid, but LPG/Propane fuelled CHPs can be bought for these situations.

Question Eighteen

Does CHP work with social housing with low electrical base load / heating profile


Andrew Gardner: Yes, as long as the site profile is analysed correctly, then a CHP unit to fit the bill can be selected.

Chris Marsland: There is likely to be a CHP like/type that will work with such a profile however the economics of it may not be viable. There is never a truly bad candidate for CHP provided they have a wet heating system; it is just that there needs to be enough energy (and the right balance between them) to make the economic model work.

Question Nineteen

Do you think CHP is suitable for every project? 


Chris Marsland: No, as I said in the webinar there needs to be a lot of detailed modelling undertaken to ensure that the CHP can deliver the promised/expected savings. There are far too many CHP installations in the UK where proper modelling was not undertaken, and the CHP simply sits idle or only occasionally runs – something that is no good for the customer or the industry.

That said there are many buildings where retrofit CHP is almost guaranteed to work economically such as hotels, leisure centres, hospitals.

Question Twenty

Can you generate electricity and feed into the grid?


Andrew Gardner: Yes, although certain criteria need to be met.

Chris Marsland: Yes, you can but there are a couple of things to bear in mind. Firstly, you have to have enough surplus electricity (once you have fulfilled the building requirements) to make it worth someone’s while to contract with you to purchase the additional electricity. This need has to be weighed against the building’s ability to use the additional heat that will be produced. It may be cost-effective to get rid of the additional heat through a dump radiator, but this will have a significant impact on the overall system efficiency.

Secondly, the amount of electricity you have available has to be reliably available to get a high-value contract and this then links into good thermal design and maintenance.

If you have any other questions you would like to ask our experts, please feel free to contact at

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