Heat Pump or CHP – which one is greener?

Faramarz Sadr

PhD

fasadr@yahoo.com

Heat pump and combined heat and power (CHP) are two low carbon technologies that are used for heating buildings in order to reduce the operational costs and CO₂ emissions. The amount of CO₂ reduction achieved depends on the emission factor of the grid. In this article these two technologies are compared with each other considering the emission factor of the grid. The results show that there is a balance point for the grid emission factor, above which the CHP produces less CO₂ and below which the heat pump produces less CO₂.

Keywords: building heating system, renewable energy, combined heat and power, heat pump, CO₂ emission, grid emission factor.

Buildings are responsible for 40% of total CO2 emissions in the world. Combustion of fossil fuels produces CO2gas which is recognized as the major greenhouse gas causing global warming. There is a strong aim to reduce CO2 emissions associated with the building sector. This mainly can be accomplished by increasing energy efficiency of the buildings and utilization of zero and low carbon energy technologies to supply the heating and electricity demands of buildings.

Heat pump and CHP are two low carbon technologies that have been proposed to be used in buildings. These technologies have been considered in the European Directive 2009/28/EC which is the main directive for promotion of renewable energy in the EU’s building sector. In addition, the Feed in Tariff and the Renewable Heat Incentive are two significant incentives that have been considered by the UK government for the promotion of renewable energy utilization, including heat pump and CHP technologies.

Electrical heat pumps use grid electricity to extract heat from a heat source. In buildings most of the CHPs are fuelled by mains gas and generate heat and electricity simultaneously. The generated electricity can supply the electrical demand of the building and displace some of the electricity consumption from the grid.

Figure 1. Predicted reduction of emission intensity of electricity (grid emission factor) in the UK according to three scenarios (2010–2030)

The other major contributor to CO2 emissions is the power sector. Under the Kyoto protocol, 37 industrialized countries and the European Community committed themselves to binding targets for greenhouse gas (GHG) emissions. In the UK, according to a report by the Committee on Climate Change (CCC), there is a need for early power sector decarbonisation in the context of economy-wide emissions reduction to achieve the 2050 target in the Climate Change Act (A net UK carbon reduction to at least 80% lower than the 1990 baseline). Specifically, the committee set out a range of scenarios for investment in low-carbon generation capacity, and proposed a planning scenario in which emissions are reduced from current levels of around 500 g CO2/kWh to around 50 g CO2/kWh in 2030 (Figure 1).This could be achieved through the addition of around 35 GW baseload equivalent low-carbon capacity through the 2020s, in addition to planned investments in renewables, carbon capture and storage (CCS) and nuclear generation over the next decade.

CO2 emissions with a gas fired boiler

Where a gas fired boiler is used to supply the heating demand of building, the annual emission rate eh can be calculated from the following equation:

                                                                                                                    (1)

Where, Fb is the fuel energy input to the boiler, H is the heating demand of the building, ηb is the thermal efficiency of the boiler and kg is the emission factor of the mains natural gas. From equation (1) the unit heating emission rate is:

                                                                                                                                   (2)

CO2 emissions with a heat pump

Where a heat pump is used to supply the heating demand of the building, the annual emission rate eb can be calculated from the following equation:

                                                                                                               (3)

where, Ehp is the electrical energy input to the heat pump, ke is the grid emission factor, H is the heating demand of the building and COP is the heating coefficient of performance. From equation (3) the unit heating emission rate is:

                                                                                                                           (4)

CO2 emissions with a CHP

Where a CHP is used to supply the heating demand of the building (in heat led operation), the emissions associated the heat supply can be reduced by the displaced grid electricity emissions. It is assumed that the CHP is fuelled by mains natural gas. The annual emission rate eh can be calculated from the following equation:

                                                                                     (5)

where, Fchp is the fuel energy input to the CHP, Echp is the electrical energy generated by the CHP, H is the heating demand of the building, ηh and ηe are the heating and electrical efficiencies of the CHP respectively, kg and ke are the mains natural gas and grid emission factors respectively. From equation (5) the unit heating emission rate is:

                                                                                                                      (6)

Grid emission factor at balance point

From equations (4) and (6) it can be seen that by reducing the grid emission factor, the unit emission of the heat pump is decreased and the unit emission of the CHP is increased. To determine at what grid emission factor (keb) these two emission rates are equal we equate the right hand side of the equations (4) and (6):

We name keb balance emission factor. The unit emission of heat pump and CHP and the balance emission factor are graphically shown in Figure 2. In this figure, the unit emission of a gas boiler is shown as well. This emission rate is constant and independent from the grid emission factor. There are also 3 interesting points in Figure 2. At point a, the emission rates of the heat pump and gas boiler are equal (). Where ke is greater than this value, the gas fired boiler produces less CO2 than the heat pump. At point b the emission rates of the CHP and the boiler are equal (), and where ke is less than this value, the boiler produces less CO2. Finally at point c (), the net emission rate of the CHP is zero.

Figure 2. Balance emission factor keb is the value of the grid emission factor ke where the unit heating emission rate eh/H is equal for heat pump and CHP

Results

Equation (6) shows that the balance emission factor does not depend on the heat and electricity demands of the building; it only depends on the COP of the heat pump and the thermal and electrical efficiencies of the CHP.

In this section we calculate the balance point of electricity emission factor for a sample heat pump and micro-CHP in the UK. We select a heat pump with a COP of 2.4 and a micro-CHP with electrical and thermal efficiencies of 0.13 and 0.79 respectively. According to SAP 2012 the mains gas emission factor is 0.214 kg CO2/kWh and the imported and displaced electricity emission factors is 0.502 kg CO2/kWh. With these data the calculated balance electricity emission factor is 0.466 kg CO2/kWh.

Currently the emission factor of the grid is 0.502 kg CO2/kWh which is greater than the balance emission factor. If a residential building has annual heating and electrical demands of 20 000 and 4 000 kWh respectively, with a heat pump the unit heating emission rate and the total emission rate will be 0.209 kg CO2/kWh and 6 191 kg CO2 respectively. With CHP these figures will be 0.188 kg CO2/kWh and 5 774 kg CO2.

According to Figure 1, in 2020 the grid emission factor will be 0.33 kg CO2/kWh. If we assume that the mains gas emission factor remains constant, the unit heating emission rate and total emission rate with heat pump will be 0.138 kg CO2/kWh and 4 070 kg CO2 respectively. With CHP these figures will be 0.217 kg CO2/kWh and 5 652 kg CO2. Figure 3 shows the balance point and Table 1 shows the summary of the calculations.

Figure 3. The balance point of a sample heat pump and CHP for grid emission factor.

Table 1. Unit heating and total emission in 2014 and 2020.

Discussion

From equation (6) we can see that the balance point of the grid emission factor depends on the emission factor of mains gas, the COP of the heat pump and the electrical and thermal efficiency of CHP. So for each heat pump and CHP, the balance point of the grid should be calculated separately. It can also be shown that by increasing the COP, the balance emission factor is shifted to higher values. Higher values of COP can be achieved by reducing the supply flow temperature of heat pump. Also by increasing the electrical efficiency of CHP the balance emission factor will be shifted to the lower values.

Traditionally in Europe the use of heat pumps is common in the countries that have cheaper electricity due to the higher penetration of renewables like wind and hydropower in Scandinavian Countries (Denmark and Norway with 0.374 and 0.002 kg CO2/kWh), or nuclear power plants (France with 0.0709 kg CO2/kWh ) or both (Sweden with 0.023 kg CO2/kWh). In the UK, which had cheaper fossil fuel, use of micro-CHP (mainly natural gas fuelled) has been common more than heat pumps.

Deregulation of the electricity market and decarbonisation of electricity can alter the tendency to use heat pumps and CHPs in different countries which should be considered by the industry.

The author would like to acknowledge Katharine Scott for her contribution in preparing of this paper.

References

CCC. 2010. The Fourth Carbon Budget: Reducing emissions through the 2020s.

European Directive 2009/28/EC. http://eur-lex.europa.eu/. Last viewed March 2014.

SAP. 2012. DECC published by BRE, Garston, Watford, WD25 9XX.