To understand how to plan for the future it is necessary to know what is happening now. We have analysed governmental data from the Department of Energy & Climate Change; collected and processed post occupancy data from our own projects and undertaken energy modelling. As a result, we have developed an in depth understanding of the characteristics of UK heat flow, including the current and predicted UK heat demand and the possible methods of supply. There are different characteristics at individual building, community and national scales.

Over several years we have studied the benefits and drawbacks of providing heat to buildings via hot water heat networks supplied from community scale heat sources, in particular combined heat and power (CHP). 

Government scenario planning includes predictions that by 2050 heat networks may supply about 20% of the UK’s building heat demand [3]. Furthermore, in London, it is currently difficult to obtain planning permission for developments without district heating and CHP. It is clear that government policy is vigorously pursuing gas fired CHP with heat networks, but to what effect? 

The issues are varied, complex and include: consideration of the heat sources that may be in use in the future; the future strategy for national electricity generation; the difference between “as predicted” and “as measured”; the relationship to the intensity of heat demand; and the costs to the end users.

The most important aspect that we have concluded is that the heat network system heat losses are very large. They are much larger than the assumed values used in regulatory and system planning calculation methods (such as SAP). 

An unfortunate feature of this (district heating) debate is that good quality data from a wide range of UK installations is not available or not publishable due to its commercially sensitive nature.  Clearly this situation is not helping the UK develop a low carbon heat strategy.

We have obtained data from the Danish District Heating Association which shows that from analysis of about 100 installations the heat losses in the municipal distribution pipes ranged from 15% to 45% of the heat supplied. This is only the loss up to the building site boundaries. There will be additional losses inside the buildings too.  The current UK average domestic heat demand is 14MWhr/dwelling/yr [11]. At this scale the Danish data shows that a heat loss of around 35%. If the heat demand from buildings is reduced to less than 10 MWhr/yr (which is desirable) then the heat losses might represent 50% of the heat supplied.   

High system heat losses (and pumping demands) mean that in many cases, gas fired CHP with heat networks will not reduce, but increase carbon emissions. This is particularly true when compared to using individual gas boilers and electricity from the current national grid.

We assert that the carbon reduction credentials of heat networks need to be reassessed (by the UK Government) taking into account the true extent of heat losses and/or the mitigation measures required to reduce them. If this is done, we may well see quite a change in national and local policies for heat networks, with or without CHP.

Our preference would be to see a much more vigorous pursual of heat demand reduction, principally by insulating and draught proofing existing buildings.

From our observations of district heating systems we believe that the very high losses can be reduced with improved components, improved design and improved care during installation. However, it is highly unlikely that the system losses could be reduced to the levels that have informed current government policy.

Electrically powered heat pumps feature heavily in government scenario planning. For example, the DECC  present a scenario with about 40% of homes and  70% of non-domestic building as being heated by air source heat pumps [3]. What are the implications of such a scenario? In 2011 we were commissioned by the Carbon Trust to find out. In general we found that there were a range of practical challenges facing a transition from gas boilers to electric heat pumps. There is an increased capital cost of the heat source and many radiators cannot adequately deliver the heat. Additionally noise is a frequent problem and there are often significant requirements to upgrade building electrical supplies.

A stark conclusion is drawn when one considers the peak power required to meet heat demand compared to that of electrical demand. The UK peak electrical demand is about 60GW whereas the UK peak heat demand is about 5 times larger, 300GW. If one wanted to change 50% of buildings to be heated by heat pumps, then one would also need double the number or size of power stations than we already have in the UK. Another option is using hybrid systems where small heat pumps meet most of the demand and gas boilers provide additional heat on cold days to meet the peaks. We have modelled such scenarios and predicted the resulting carbon benefit and power station requirements.  

It is true that as grid supplied electricity becomes decarbonised heat pumps become more carbon efficient. Currently the UK grid carbon intensity is around 550g /kWh whilst many government planning scenarios predict an intensity of 50gCO2/kWhr by 2050. However, achieving a 50g grid carbon intensity whilst increasing the installed capacity multiple times is no mean feat! Such a change faces formidable technical, environmental, political, economic and social challenges.

Nearly all of the heat used in the UK is used to heat houses and most of this heat is leaking away through walls, windows, roofs and draughts.  We believe that in order to achieve a low carbon future, addressing this vast heat loss should be a priority.

We have collated and analysed published data from the Retrofit for the Future project [6] and the Low Energy Buildings Database [10], in which several existing houses were provided with low energy renovations. The data is incomplete, however we have managed to extract some useful conclusions.

The space heat demand for the post retrofit houses ranged from 10 to 60 with an average of 30 kWhr/m²/yr. No information was available for the pre retrofit situations, but analysis of national data [4] shows that the national average dwelling space heat demand is around 130 kWhr/m²/yr. The insulation costs ranged from £130-900/m2 with an average of £400/m2. For comparison, the cost of constructing a typical new house is about £1300/m2. So it appears that insulation, along with draught proofing and in some case MVRH can provide significant energy savings but are currently quite expensive.

In addition to the reduction in energy demand and the associated CO2 emissions, reducing the heat demand of buildings has other very important benefits. Insulated houses are likely to be more comfortable and healthier places to live. Reducing the peak heat demand makes meeting that demand, from renewable sources, much easier. In the case of heat pumps, the more insulated houses there are, the fewer power stations are required.

Reducing the UK heat demand (by insulating and draught proofing all existing buildings) is certainly beneficial, however some current government policies actually discourage it from happening. District heating is being pushed by policy whilst the economic case for district heating is most favourable in cases with high heat demands, that is, cases where buildings are uninsulated and draughty. Furthermore, the Renewable Heat Incentive (RHI) government subsidy scheme which promotes installations of low carbon heating technologies such as heat pumps, biomass boilers and solar thermal water heating provides the highest revenue for owners who sell and use as much heat as possible.  One may argue that promoting district heating and the RHI  not only diverts money away from insulation but also acts as a counter incentive to reduce the heating load.

A critical aspect of a transition from fossil fuels to a renewable energy future is the ability to store energy for use at times when the sun is not shining or the wind is not blowing.  Available techniques are numerous and include: pumped hydro, chemical batteries, compressed air stores and underground thermal energy stores.  Another possibility is to use renewable electricity to make hydrogen or methane that can be stored, distributed to the points of demand and used when it is needed.  A benefit of using gas (synthetic natural gas or hydrogen) as the method of large scale heat energy storage is the fact that the storage vessel and transmission system already exist, namely the national gas grid.

We have undertaken numerical modelling of a set of scenarios, all with large scale renewable electricity generation by offshore wind turbines, to provide enough power to heat all the buildings in the UK. In these scenarios wind generated power can be used to heat a building base load via heat pumps, or can be converted to hydrogen which is stored and used in fuel cells to meet peak heat demands.  The diagram below illustrates the principle. 

Our simulation model uses National Grid data of current UK wind power output with an expansion factor to predict future UK wind power hourly generation potential.  We have made our own model of UK hourly heat demand which incorporates daily demand profiles and the dependence on external temperature.

We have used the model to calculate the extent of wind of power that would be required to meet the UK heat demand for different heating strategies. The model is fairly crude but does account for the key issue which is the fact that the times of high wind generation potential do not always coincide with the times of high heat demand. Being able to store energy (even inefficiently) can reduce the amount of wind generation capacity required to meet the heat demand.    

The future solution to deliver low carbon heat is inextricably linked to the futures of transport and electricity generation. These three major energy demands can both compete with and complement each other with regards to meeting the demand. For example, if plenty of low carbon electricity is available for transport and heat, then heat pumps will be a good option. If electricity is not decarbonised then they will not. Either way, reducing demand will always be beneficial.

We feel that the energy, carbon and peak load reduction benefits of reducing heat demand by insulating and draught proofing all existing buildings has been undervalued (by policy makers). We acknowledge that currently costs are high but we think that much more research and development should be being undertaken to address the technical, social and political challenges, with a view to reducing the costs.

National Grid [7] have recently published a report describing several scenarios to drastically reduce the carbon emissions associated with electricity production. In these scenarios the key components are wind power, nuclear power and carbon capture of fossil fuel combustion emissions. The scenarios are similar to the “2050 Futures” described by DECC [8]. None of them are trivial to deliver!

If the electricity grid can be decarbonized heat pumps will be a good option to produce low carbon heat most of the time. This will be effective at reducing fossil methane (natural gas) use and the associated carbon emissions, however meeting peak heat demands will still be an issue. The more buildings are insulated and draught proofed the more effective heat pumps become and the fewer power stations (of any type) are required to power them. Conversely, district heating incurs huge heat losses and so increases the amount of heat power required compared to having individual heating devices within each building.  To meet peak heat demands it would be useful to maintain the existing national gas grid. In time we could make gas itself from the decarbonized electricity.  The process of making hydrogen and methane from electricity is well understood but does loose energy in the process, so it is likely to be the last phase of decarbonizing heat. That said; there is an ambition to decarbonize road transport using electricity. This will involve storing electricity in batteries or producing hydrogen for the vehicles to run on. Consequently, an infrastructure of hydrogen for transport may be a viable scenario sooner and this drive could facilitate the use of hydrogen for heat.  The fuel cells in cars could also be used in buildings to produce electricity and heat, which could ultimately displace the need for central power stations.