Heat Pumps for Gas Boilers?

Posted by Paul Spare on 31 May 2023 in Articles

Tagged with: Climate Change Committee, Gas, Heat pumps.

It has been government policy for many years to reduce UK CO2 emissions. The greatest contributors are electricity generation; transport and domestic gas use. The first two have received considerable attention. The Climate Change Committee (CCC) has recommended the main thrust for the domestic sector will involve replacing domestic gas boilers by heat pumps, installing at least 600,000 each year by 2028, increasing to 1.9 million per year by 2035.

It is proving difficult to persuade the public to replace their efficient and convenient gas boilers because generally, heat pumps will not have equivalent thermal performance. This is a blessing in disguise, because providing the electrical energy for the new equipment will be a massive challenge.

There are two considerations – generating the TOTAL power and also having the capability of increasing output at a RATE to match changing demand.

Electricity is generated and despatched via a national control system, because supply and demand have to be matched continuously within tight limits. The lowest base load is about 20,000 MW in the early hours of a Sunday in mid summer. Maximum demand of 50,000 MW occurs in December or January, but must be within the capacity of the generating plants although it may last only a few hours. The variable electricity demand is shown as the red line in Figure 1. The early morning electricity surge has a maximum of about 20,000 MW over 3 hours (i.e. a rate of 7,000 MW/h).

Domestic gas demand (It is given the term “Non-daily metered gas demand”) is not controlled centrally and the variation over the year is not measured directly, but an analysis that was undertaken ten years ago shows typical pattern (also shown in Figure 1).

Non-daily metered gas demand has an average of 1.5 TWh/day in the winter, with a range of 0.5 to 3.5 TWh/day. Expressed as power, this gas supply equates to a winter daily average of 100,000 MW, allowing for the efficiency of gas combustion. The demand through the day is very uneven, being concentrated in the early morning and evening peaks. There is no precise data, but the peak is estimated from Figure 1 to be at least 150,000 MW. There is thus an increase of 100,000 MW over about two hours on cold winter days (rate of change of 50,000 MW/h). There is a small ameliorating factor. Heat pumps extract energy from the surroundings according to their Coefficient of Performance, so the demand increase may be reduced to 25,000 MW/h rather than the full 50,000 MW.

Even during the last winter energy crisis, both the national electricity and gas demands were accommodated by exploiting the different qualities of the energy supplies.

The early morning electricity surge is currently met by powering up 20 - 30 CCGT plants over three hours. The increased gas demand is met by depleting underground storage facilities and lowering pressures. If switch from gas-fired boilers to heat pumps is achieved, the heating load that is supplied by natural gas will be transferred to the electricity grid, producing a very substantial new source of demand. The storage benefit provided by gas will be lost. Electricity generation has no such substantial storage/reserve. Electrical supply must be matched to demand within seconds. The rate of change of heat demand is still almost four times the maximum rate of increase achieved in our electricity supplies, so the plant to provide heat energy must still be quick acting.

The CCC have been alerted to this risk. They commissioned a report in 2013 (Reference 1) to examine the problem of replacing gas heat by electricity However, the CCC have taken little account of this in making their proposals for domestic gas. Reference 1 noted that whereas in 2011, the daily range of electricity consumption varied from 0.675 TWh to 1.2 TWh, the domestic component of gas varied over the range 0.368 to 3.49 TWh, and would dominate the early morning surge in electricity demand.

Renewables cannot respond to demand so meeting the gas demand that is transferred to electricity might require 100 – 150 new CCGT/OCGT plant. The use of CCGT plants makes sense only if the CO2 is captured from the waste gases, although capturing only 80% may be feasible, with a considerable loss of efficiency.

Could nuclear plants provide this function and respond at a fast enough rate to meet the increase in electricity demand? Although nuclear plants in the UK have traditionally operated as permanent base-load, there are solid grounds for optimism. The OECD commissioned a paper in 2011 examining the operations of reactors in the EU (Reference 2) .

Modern nuclear plants with light water reactors are designed to have strong manoeuvring capabilities. Nuclear power plants in France and in Germany operate in load-following mode, i.e. they participate in the primary and secondary frequency control, and some units follow a variable load programme with one or two large power changes per day. The minimum requirements for the manoeuvrability of modern reactors are defined by the utilities requirements that are based on the needs of the grid operators. For example, according to the current version of the European Utilities Requirements (EUR) the NPP must at least be capable of daily load cycling operation between 50% and 100% of its rated power Pr, with a rate of change of electric output of 3-5% of Pr per minute.

Most of the modern designs implement even higher manoeuvrability capabilities, with the possibility of planned and unplanned load-following in a wide power range and with ramps of 5% Pr per minute. Some designs are capable of extremely fast power modulations in the frequency regulation mode with ramps of several percent of the rated power per second, but in a narrow band around the rated power level. This is an increase in demand for just a few weeks in the winter (for only two hours per day) and must be met by generation plant that can be relied upon or we risk lengthy power cuts.

The construction of new nuclear reactors to provide this peak power will still be wasteful because they would be needed for so few hours each year, but if the primary consideration is the reduction of CO2 emissions, with costs only a secondary factor then increased nuclear investment can be justified.

  1. Grant Wilson et al, Energy Policy vol. 61, 2013, pp301 – 305. (Ref.3)

  2. Technical and Economic Aspects of Load Following with Nuclear Power Plants

Great Britain energy vectors daily demand - TWh Gas vs Electricity 29th September 2010 - 28th January 2013 (28 months)