The future of residential heating
Space heating. The Netherlands has approximately 7.8 million houses, of which more than 85% are heated using central heating boilers. About 19% of the total Dutch CO2 emissions can be attributed to these houses. Nearly two-thirds of these CO2 emissions are attributable to space heating and hot tap water. The rest is attributable to electricity generation for residential use. This makes CO2 emissions for space heating of houses and tap water the largest contributor of CO2 emissions from households.
New residential building. The CO2 emissions for space heating of new buildings are very limited in proportion. In relative terms, the annual number of new homes is limited (0.8% growth in the housing stock), and these new homes have a low heat demand due to good insulation. This low heat demand subsequently makes it possible to use low-temperature underfloor heating. As a result, regardless of which generation method, the heat demand and associated CO2 emissions are low.
Existing residential buildings. Therefore, the greatest savings on space heating must be achieved in existing homes that are now largely heated with gas-fired boilers and high-temperature (HT) heating systems. After all, these homes have a much greater heat demand than new construction homes and will still be the bulk of the housing stock in the coming decades. This is, therefore, the major challenge, both economically and technically. First of all, where possible, the heat demand will have to be reduced through improved insulation and/or (expensive) renovation of the housing shell. In addition, methods to generate heat with less CO2 intensity will have to be looked at for each type of home or area.
CO2 emissions from various heating solutions
Stop using gas. Various stakeholders have suggested shutting off homes from gas in the coming years and heating them in other ways. In densely built-up areas, heat networks can sometimes be used for this purpose if (industrial) residual heat of sufficient quality or heat from efficient CHPs is available in the area. However, this only applies to a limited part of the existing building. In addition, electric heat pumps are often mentioned as a sustainable alternative to high-efficiency boilers. To assess the CO2 savings of this, it is useful to compare the CO2 intensity of the heat generated by electric heat pumps with that of modern condensing boilers and with that of the thermally driven heat pump.
CO2 intensity of heat. A modern condensing boiler supplies heat with an average CO2 intensity of 1.89 kg CO2 per m3 gas, or 0.20 kg/kWh heat. However, for an electric heat pump, it is slightly more complex. The CO2 intensity of the heat can be calculated by dividing the CO2 intensity of the electricity used by the year-round efficiency (SCOP = Seasonal Coefficient of Performance) of the heat pump. Both these numbers are crucial for determining the CO2 intensity of electric heat pumps, but at the same time, there are many misunderstandings about them. Therefore, some explanation follows below.
The CO2 intensity of the electricity mix from the grid is published annually by Statistics Netherlands (CBS) , using two numbers: the average emission factor (integral method) and the marginal emission factor (the ‘referentiepark’, or reference park, method). The average emission factor is based on the total electricity production (renewable plus non-renewable). The marginal emission factor is based on electricity production from fossil fuels and nuclear energy. To analyse the effect of a change in the electricity production system due to savings (e.g. installation of solar panels) or additional consumption (e.g. shift from gas boilers to electric heat pumps), the marginal emission factor applies. In other words, installing solar panels lowers the required centrally fossil-generated electricity, and installing electric heat pumps increases the required centrally generated fossil-generated electricity by the same factor. This is the case as long as there is no surplus of renewable electricity, and this will, unfortunately, be the case for decades to come – especially during the heating season. The figure below shows the development of the marginal emission factor in the Netherlands. The drop in 2016 is caused by the closure of three old coal-fired power stations. The current level is expected to decrease slightly after 2030, towards 0.50 kg/kWh, if a few more coal-fired power stations are closed.
SCOPs of electric heat pumps. These numbers can be used to calculate the minimum SCOP that an electric heat pump must achieve now and in the future in order to achieve a lower CO2 intensity than the high-efficiency boiler. At the moment that is approximately:
In the future, after the closure of the coal-fired power stations, this value will drop to approximately 2.5. For high-temperature (HT) central heating systems, electric heat pumps achieve a SCOP of maximum 2.5. Such returns are still very attractive if the electricity is generated almost entirely sustainably (Scandinavia) or with nuclear energy (France), but in the Netherlands, this is insufficient to compete with the condensing boiler on CO2 emissions. For that reason, low-temperature (LT) underfloor heating systems are usually also used in the Netherlands in combination with air-water heat pumps. However, installing such underfloor heating systems in existing homes throughout the house usually requires very costly home modifications. A recent article by the renowned German Fraunhofer research institute provides important conclusions about the measured performance of low-temperature (LT) air-to-water heat pumps in practical situations (often installed by manufacturers themselves) :
- Very best results: SCOP = 4.2
- Average resultats: SCOP = 3.2
- Worst results: SCOP = 2.2
This concerns systems that were installed after 2012, the results in the years prior were worse. The above numbers only apply to space heating, so without the preparation of hot tap water! Due to the required high temperature (60°C) of hot tap water, the total SCOP of the total heating system drops, usually by about 0.5. However, this strongly depends on the user situation: the ratio between the amount of tap water and space heating. This means that the best LT heat pumps in practice achieve a maximum SCOP = 4.0, but that for most LT heat pumps the SCOP is around 3.0 in practice, including tap water preparation. The above numbers correspond to rules of thumb from the professional installation sector: a properly installed LT heat pump system can now achieve SCOP = 4.0, but SCOP = 3.0 is more often the case, including tap water preparation. Conclusions comparison of CO2 intensity. The above means that the average CO2 emissions of LT heat pumps are at a similar or even slightly higher level than the emissions of a condensing boiler. In other words: as long as the coal-fired power stations in the Netherlands are not closed, it makes little sense in terms of the CO2 emissions to introduce large-scale LT electric heat pumps in existing construction, let alone HT heat pumps. With well-installed systems, only a small profit can be yielded – at a high price. In the diagram below, the CO2 emissions of the generated heat from the various technologies are compared, standardised relative to the condensing boiler.
Some comments can be made about the above argumentation:
- This argument applies as long as there is no surplus of sustainable electricity on the Dutch electricity grid during the heating season. As far as is known, this will certainly be the case well beyond 2030.
- The above reasoning does not include additional losses in the electricity grid as a result of peak loads from electric heat pumps. Grid operators expect that the electricity grid will experience above-average losses when many heat pumps are running at the same time, so that marginal CO2 emissions at that time are in fact higher than the annual average numbers.
- Compared to the condensing boiler, hybrid heat pumps can reduce CO2 emissions because the heat pump part can run with a slightly more favourable SCOP during the less cold periods of the season. In the cold periods and for hot tap water, the condensing boiler is relied on.
- Water-water LT heat pumps with a bottom heat exchanger can significantly reduce CO2 emissions. According to the same article by the German Fraunhofer research institute , the latest systems deliver SCOP = 4.3 on average; the very best systems deliver SCOP = 5.4. This means a 27% and 42% reduction in CO2 emissions compared to the condensing boiler. However, this solution is not easily applicable in existing construction.
- M. Miara, 10 years of heat pumps monitoring in Germany. Outcomes of several monitoring campaigns. From low-energy houses to un-retrofitted single-family dwellings. 12th IEA Heat Pump Conference 2017 (pdf).