By Dave Elliott
The current decarbonisation plan in the UK is to replace the use of fossil gas for domestic heating by the use of electricity. That may have been slowed by the abolition of the Zero Carbon Homes policy for new build, with its 2016 deadline, but there is still a strong push in that direction, with a ‘Near Zero Energy in Buildings’ concept and specific programmes for roll-out of technologies like electric heat pumps, apparently one of the government’s favoured heat supply options, along of course with insulation and improved building design to reduce demand. The latter makes sense, the former, well that is debatable.
Heat pumps do use electricity efficiently to pump low-grade ambient heat, captured from the air or the ground (or, in some locations, water) into the house, with a coefficient of performance (‘COP’) of around 3. So you can get 3 units of heat for one unit of electricity. But not always. In the winter, when the temperature differentials are lower, air source heat pumps are less efficient. The heat absorption pipes can also frost over: unless it is cleared off, by some sort of defrosting system (or manually!), the layer of ice will reduce heat transfer efficiency. Heat pumps, whatever the type, also have to be carefully installed and regularly maintained if high COPs are to be consistently sustained. Otherwise you might as well use the electricity in a standard resistance heater. Or indeed use gas, in a cheaper smaller high-efficiency combi-boiler. Except there won’t be much gas to use for that purpose, under the new plan.
That will be a big change. Around 90% of homes are heated mostly by gas at present. It makes a lot of sense, since it is cheap and lower carbon than electricity produced using coal. Electricity produced using gas may be a bit better, except, at best, the gas-to-power conversion process is around 50% efficient and there are losses in transmission and distribution, maybe up to 10%. By contrast, gas transmission is low loss and the gas pipeline acts as an energy buffer store. There are also large gas storage units. It is very much easier to store gas than electricity, and the gas system provides a flexible way to deal with the big swings in demand for heat that occur. These swings are much larger than those for electricity, in winter especially. Not only is the scale of heat demand much larger than that for electricity, but the variations in demand, each day, and over the year, are also very much larger. The gas grid system is designed to cope with that. If it is abandoned, and we try to use electricity instead for heating, there may be real problems. At present the gas grid handles about 3 or 4 times more energy than the electricity grid. So the electricity grid, nationally and locally, will have to be expanded significantly. So it’s not just the problem of installing heat pumps in every house (that can be quite disruptive). Local distribution grids will also have to be upgraded, which can be even more disruptive.
Note that this problem will get even worse if, as planned, there is a switch to electric vehicle (EV) use. The vehicles’ batteries will need charging and that will mostly be at night after work. One study suggested that peak electricity demand could rise 20% with large-scale EV charging. So the grid will be even more strained: www.gov.uk/government/uploads/system/uploads/attachment_data/file/48191/3150-final-report-changing-energy-use.pdf
Estimates of costs vary. One study has suggested that the necessary local electricity grid reinforcement needed to handle the heat pump load would account for 60% of the cost of upgrading the system for low carbon technologies, most of the rest being for EVs. www.gov.uk/government/uploads/system/uploads/attachment_data/file/370648/Final_Report_-_Impact_of_Policy_that_Drives_Low_Carbon_Technologies_on_Distribution_Networks_.pdf
What is the alternative? The most obvious option is district heating – heat supply networks fed with heat from high efficiency Combined Heat and Power (CHP) plants. Although domestic heat pumps can have a coefficient of performance (COP) of maybe three, larger community-scale CHP plants, using otherwise wasted heat, have COP equivalents of up to 10 or more, depending on the network temperature required. Installing district heating networks only makes sense in urban or some suburban areas, and heat pumps can make sense in off-gas-grid areas, but there are clear advantages with larger community-wide district heating systems, as used widely across much of northern Europe: http://setis.ec.europa.eu/system/files/JRCDistrictheatingandcooling.pdf
In a 2012 report, the Royal Academy of Engineering said that ‘larger district systems, incorporating a CHP facility and providing heating are significantly more efficient than domestic-level installations. Central systems may be more efficient and are likely to offer much greater energy storage than do systems designed for individual households.’ www.raeng.org.uk/heat UK interest in CHP/district heating has now expanded, with DECC backing Heat Networks: ‘some models show technical potential to supply as much as 43% of heat demand for buildings through networks by 2050’ www.gov.uk/government/publications/delivering-uk-energy-investment-networks-2014
However, the plan to phase out gas for heating still remains. There have been some rumblings of discontent. A study by a team from Imperial College, echoing the RAe’s conclusions, noted that compared with domestic-scale electric heat pumps ‘Shared heat systems, due to economies of scale, can be much more efficient in the production of heat, can significantly lower electricity system peak loading’, and ‘may also more readily incorporate options for storage which can reduce peak demands’. There were also system integration benefits since CHP plants can vary their heat to power output ratio in response to varying availability of renewable power: ‘the amalgamation of district heating with CHP and renewable electricity generation can prove a valuable means of balancing supply and demand across the heat and power sectors’. https://workspace.imperial.ac.uk/icept/Public/Energy%20System%20Crossroads.pdf
The UK Energy Technologies Institute has also been looking at heating and has warned that ‘a consequence of a low carbon transition is the significant reduction in the flexibility of supply provided by gas heating systems.’ It also noted that there would be residual costs with supporting the remaining gas system, to the extent that if it survived: ‘the inherent fixed costs of the gas network would…need to be met by the diminishing consumer base’. It did not come down on the side of either of the main heating options, suggesting that they might end up supplying about the same amount of heat each by 2050, but did say that ‘heat pumps are generally most effective when delivering constant background heat rather than producing rapid temperature changes’, while ‘heat networks could become the system of choice for many UK consumers as they have in a number of Western European cities’. www.eti.co.uk/wp-content/uploads/2015/03/Smart-Systems-and-Heat-Decarbonising-Heat-for-UK-Homes-.pdf
There certainly is a good case for heat networks. Although installing heat mains in the street is disruptive, once that has been done, they can be fed with heat from whatever source is best, with, as the Imperial team put it, change-overs involving ‘no disruption to individual users fed by the heat networks’. Most CHP plants at present use fossil fuel, usually gas, but biomass (e.g. straw) is increasingly common and biogas could become significant along with synthetic gases from various sources, including gases made using surplus electricity from wind plants. Solar heat is also an option: Denmark plans to get 40% of the energy for its extensive district heating network from solar by 2050, with large interseasonal heat stores saving summer heat for winter use. Whether heat networks should replace gas networks remains an issue: gas transmission is much easier over long distances than heat transfer and green gas options may proliferate. Large gas-fired heat pumps are also an option! All of this arguably being better than relying on electricity for everything. However, as the ETI says a little wistfully, ‘the interaction between energy vectors seems set to increase in future systems with a need to integrate the operation of the gas, electricity and heat sectors’, but ‘there is currently no owner for the holistic view of integrated electricity, gas and heat systems’. www.eti.co.uk/heat-insight-decarbonising-heat-for-uk-homes/