Advantages of Combined Heat and Power – CHP – Cogeneration
Advantages of CHP
Heat is generally a by-product of generating electricity: this is always true if generation is based on combustion of carbon fuels (like coal, oil, gas or biomass). The principal advantage of locating electric generation close to buildings is that the heat from generation can be used to heat local buildings.
Although all combustion of carbon compounds releases CO2, a larger proportion of the inherent energy in the fuel is converted into electricity or useful heat in a "Combined Heat and Power" plant ("CHP").
However, CHP only saves carbon emissions for heating when there is a simultaneous need for electricity and heating. This may be true in winter, but is unlikely to be the case in summer.
With the rapid decarbonisation of the Grid, electric heating using heat transfer is now a more secure route to the decarbonisation of heating – and cooling.
Disadvantages of CHP
However, there are strong reasons why district energy schemes based on CHP have not taken off in the UK and there will continue to be substantial barriers:
- installation of a CHP requires large up front capital investment
- it requires space for the CHP "energy centre"
- it requires large diameter heavily insulated metal piping for the hot water network
- it suffers heat losses to the ground
- set up costs to administer and run the central "energy centre" over the whole life of the system
- set up costs to run an accounting system to charge tenants and collect substantial fees each quarter
- risk that clients may not want to sign long term heat contracts
- risk that if some clients fail to meet their obligations, others may need to contribute more
- many modern buildings need cooling as well as heating
- the heat generated in summer may go to waste
- it is not practical to flex the scale of CHP to meet changes in demand, or expansion of the network
- most organisations want to be in control of their own costs
- combustion within cities is a major cause of air pollution and premature deaths.
Luckily there is an alternative form of heating which neatly resolves all the limitations of CHP based district heat networks.
Heat Sharing – Community Heating – Heat Networks
There is an alternative means of sharing heat using a lower temperature distribution circuit. This heat sharing network, which is linked to a communal ground array can be accessed by each building on the network: each building uses its own heat pump to raise the temperature to the temperature it requires for its own heating and hot water requirements (or rejects heat to the network if it needs cooling).
A heat sharing circuit is much cheaper to install than a high temperature circuit because it does not require insulation to prevent heat losses to the ground. In fact, heat exchange with the ground that the pipes pass through can be beneficial: the ground adjacent to the pipes extends the contact with the ground and the pipes can draw heat from the ground.
Buildings with excess heat can reject heat to the Heat Sharing Network. This heat exchange with cold water is much more efficient than rejecting heat to hot air. It also raises the temperature of the Heat Sharing Network for the benefit of those buildings which need to extract heat.
Heat Sharing based on a Heat Sharing Network can ...
- provide a reliable and low-cost green energy source for space heating and cooling
- save over 70% on carbon emissions on heating compared to emissions from gas boilers
- save over 80% on carbon emissions on cooling buildings compared to emissions (from the power stations) that power electric air conditioning and electric chillers
- provide a low-cost heat energy (or cooling) source for industrial processes
- recover low grade heat from any local source to increase the efficiency of heat pumps on the network
- provide opportunities for reducing carbon emissions by re-cycling solar energy instead of burning fossil fuels
- provide the opportunity to recover heat from buildings with high occupancy and high passive heat gains and transfer it to buildings needing heat
- attract Renewable Heat Incentive for use of ground source heat pumps
- attract Renewable Heat Incentive for use of solar thermal collectors
- improve urban air quality by avoiding combustion of fossil fuels or biomass in densely populated areas.
Groups of Buildings – District Heating
A Heat Sharing Network is very well suited to providing heating and cooling to groups of buildings, particularly buildings under common ownership, including on hospital or university campuses. The cost of providing an efficient installation can be shared across a number of buildings, and the benefits increase if the district heating system includes other buildings such as schools or offices whose heating and cooling requirements may follow a different daily pattern (and different weekly pattern) from the heating demand for other buildings on the network. Where the district covers offices, or data centres, the heat recovered from cooling these buildings can be transferred to homes requiring heating (or other buildings with a heating need such as a community swimming pool).
Where the cooling demand is separated in time from the heating demand, surplus heat can be stored in ThermalBanks from the time it is available to the time it is needed. This efficient use of heat is at the heart of Interseasonal Heat Transfer and enables ICAX to provide cheaper heating and cheaper cooling than conventional methods, as well as proving heating and cooling with a very low carbon footprint.
Sharing heat between buildings
Many buildings have an overall cooling load over the year: they have a requirement to reject heat. This often applies to modern office buildings in south east England with extensive glazing and high solar gains. These buildings may be adjacent to older buildings with an overall annual heating load. ICAX has developed systems to allow for the transfer of heat between buildings: this form of heat transfer can save fuel and carbon emissions for both buildings.
Both buildings can benefit from a "heat sharing dividend" when they enjoy "joined-up heating and cooling".
Other buildings with a need to lose excess heat include underground train tunnels, data centres and supermarkets.
A comparison of the advantages of Heat Sharing Networks over traditional gas powered District Heating is shown in the Heat Sharing Table.