Switch2 Blog

On the fence for CHP

Posted by Ian Allan on 09-Dec-2016 11:39:46

Social housing and private developers are faced with the decisions on what type of renewable energy to put into their community heating plant rooms. In many cases Combined Heat and Power (CHP) is the obvious choice. But what should you know about CHP?


What is CHP?

Simply put, CHP is an engine that is connected to an electricity generator - but instead of the heat from the engine being wasted, infast up to 60 per cent of the energy used to generate electricity goes up the chimney in a normal generator or power station; the heat in the exhaust gases and engine cooling system is harvested and put to good use in a building heating system.

CHP units have been in operation in the community heating plant rooms since the 1970’s; and is a process that produces both space/water heating and electricity simultaneously. However, it has not always had a good reputation mainly because the CHP has been oversized and not well maintained, this has led to CHP engines being switched off and left unused. Installations are a high CAPEX investment and can be complex to operate. Therefore you need to be absolutely certain it is correct for your scheme and is going to benefit your residents, not just a tick in the box for planning.

To get get maximum benefits and return on investment (ROI) you need to consider the following:

  • Your CHP engine should run 365 days, and at least 17 hours per day
  • The system must be designed to use all the heat and electricity generated. To do this the engine should be sized for the heat base load, often referred to as ‘heat lead CHP’
  • Ideally electricity generated should be used on site and not spilt to the grid at a low price
  • You need to run full capacity from day one. There is no point in having a high CAPEX plant sitting idle in your plant room or working at very poor efficiencies

What is a heating base load?

A heating base load is the minimum level of demand needed to keep the CHP over in full operation at full capacity for 17 hours per day. By establishing the base load a scheme operator can then quantify how much heating and power is needed in order to meet the minimum requirements for the scheme. Simply, thermal stores are an option to smooth out demand and potentially keep your CHP running longer.

More on what you do with electricity generated

Electricity can be sold back to the grid, however they will only pay between 2-4p kWh. But if you use the electricity on site, you will be displacing electricity that would have been purchased at roughly 12p kWh. This can make a significant difference on the return on investment. Traditionally this would be used in landlord electricity supplies, such as communal lighting and plant room pumping.

What is the fuel for your CHP?

Although Biomass is available, most small scale CHPs use natural gas.

For every kWh of CHP how much do you get out?

To get to the nub of why CHP works, for every 100 kWh of gas that goes in, you get out 33 kWh of electricity and 52 kWh of heat and 15 kWh are wasted. This is far more efficient way of generating high value electricity than the traditional way of producing electricity where the heat element is wasted.

CHP Sankey Diagram

Operations of a CHP plant

Once you have installed a suitable sized CHP into your energy centre it is important that is is monitored and maintained correctly if you are to benefit from the expected ROI. The maintenance of CHP is costly so it is important that it ensures your system is running reliably and efficiently.

Positive news for efficient CHP systems

If you are successful in implementing an efficient CHP system, then you will be eligible for the Government's CHPQA Programme. This offers tax incentives for schemes that meet its basic requirements, which makes the technical requirements of CHPQA an important factor to consider when designing a scheme.

Steps to specifying successful CHP

Step one:

Get an expert on CHP for communal heating schemes to help you. However it will really help if you understand the basic principles.

Step two:

The minimum level of heat and electricity demand (usually called the base load)  is critically important when considering a CHP installation. This is due to the fact that a high level of utilisation is vital if the installation is to be operated efficiently. Normally a utilisation of 17 hours a day 365 days a year is the target.

Step three:

You need to consider the seasonal variations of heat and power. Think about how you will operate a CHP plant in both winter and summer - will you need to utilise thermal storage? Thermal storage can be a way of extending a run time of your CHP.

Step four:

Any plans that might change the consumption of energy should also be considered so that the CHP plant that is installed does not end up being either insufficient or excessive for the requirements of the site.

Step five:

Consider options for funding; either you buy the CHP outright, and arrange the monitoring and maintenance yourself, or there are CHP companies that will fund the capital and operation based on selling the energy to you at a discounted rate. The latter may be worth considering if you do not have access to expertise or the time to monitor the operation yourself.

There are too many under-utilised CHP schemes that have been included in developments purely in order to tick boxes. However, if taken seriously, CHP schemes can be the single biggest cost-saving measure that is applied to a development. For this reason, it is worth carefully considering how they can help you and the residents in your properties.

Key takeaways:

  • CHPs can be a real asset  if implemented and maintained correctly
  • Think about who will benefit from the CHP once installed- will the CHP engine reduce heating costs for the resident?
  • On CHP community heating system there should be no waste heat produced as it should be sized correctly
  • Do not confuse CHP for community heating where it should be heat lead with larger scale industrial electricity lead projects

Best Practice in Action - how to implement the CIBSE code of practice


Ian Allan

Head of R&D at Switch2