Total carbon dioxide emissions from ground source heat pump and groundwater one in Białystok

. To limit greenhouse gases emissions caused by energy production European Union (EU) prompts heat pump as heat generator which should decrease CO 2 emissions to the atmosphere. Because of the climatic conditions and low efficiency of electrical energy production and transfer in Poland it could be possible a condensing gas boiler would emit less CO 2 . The analysis includes ten-year temperature measurements in Białystok where is more severe climate in Poland. Due to relatively high seasonal coefficient of performance (SCOP) value heat pumps can emit less CO 2 than condensing gas boiler and can be applied as ecological heat generators.


Introduction
Temperature rise which has been observed on the land part of the Earth at the end of the second millennium, started a discussion its origin. By a common agreement of the majority of the states the greenhouse gases (methane, carbon dioxide, water vapour, ozone, nitrous oxide, halogenated hydrocarbons, etc.) emissions are blamed global warming. Isasmuch as, carbon dioxide concentration has been increasing from 316 ppm [1] at 1960 to 411.69 ppm on 1 st April 2019 [2] the limitation of CO2 emissions by minimum 20% of the 1990 content before 2020 year is a target of European Economic Area states [3]. Thus, EU promptes the exchange heating devices that combust fossil fuels for heat pumps [4].
Although heat pump emit no gases in its location, it uses up electrical energy which is generated in Poland from fossil fuels burning. Moreover, efficiency of electrical energy production in Poland ηp = 37.6%, as stated by GUS [5], and efficiency of low voltage energy transfer t = 79.2% which was fixed by Ciura [6]. Overwhelming majority of electrical energy in Poland origins from bituminous coal and lignite combustion, see in Table 2. If we consider efficiency of electrical energy production and transfer, heat pump in Poland is energetically profitable if seasonal coefficient of performance SCOP > 3.5 (cf. Rubik [7], Gajewski et al. [8]). It means, a heat pump which SCOP equals 3.5, generates so much total (in situ plus distant) CO2 emissions as a coal fuel boiler. Thus, it was possible the heat pump would cause more CO2 emissions than a condensing gas boiler (cf. [8]), which is verified using an inequality derived in the paper [8].
SCOP kgCO GJ (1) where: a -average value of generation factor for electrical energy production [kgCO2 /GJ], g -green value of generation factor [kgCO2 /GJ], ng -generation factor for natural gas combustion [kgCO2 /GJ], b -efficiency of a condensing gas boiler. Both a and ng include direct and indirect emissions. If inequality (1) is met then the heat pump is ecologically viable. Otherwise, the condensing gas boiler should be applied. The aim of research is estimation of emitted CO2 amount to atmosphere during ground to water heat pump (GSHP) and water to water heat pump (WSHP) operation. The first step is determination of seasonal coefficient of performance of GSHP and WSHP, which was done in the work [9]. The second stage which is total CO2 emissions estimation, is presented in the paper.

Computations
Although an algorithm origins from European standard [10], because of buffer tank operation it must have been changed so as to accurately estimate the heat losses. As the buffer tank is dumped and unloaded, the computations start at midnight on the first day of a heating season and are continued in the consecutive hours until the end of the heating season, which is necessary modification of the standard's algorithm [10]. For that purpose, the mean values of external temperature are calculated in every hour (see in  There are two alternative heat generators: GSHP or WSHP which the most important data are written down in Table 1. If groundwater composition did not meet the manufacturer's requriements the WSHP circuit should be disjointed from groundwater by a separating heat exchanger (SHE). Thus three variants are taken into consideration: GSHP, WSHP and WSHP+SHE. In every variant heat pump have to be separated from a heating system with a buffer tank (cf. [11]) generating additional losses Qj.
where te is external design temperature, ti is internal air temperature, tj is bin temperature (external air temperature). Electrical energy consumption is predicted using an expression that is extended from the equation in the standard [10] to preseneted hereafter formula that includes heat losses of buffer tank Qj: where COPbin(tj) is obtained from the manufacturer data [11]. Whereas Qj. is computed in every hour from a relation: To correlate bin temperature tj with temperature tk in the buffer tank the latter is obtained from heating curve [11] which is approximated by quadratic polynomial shown in Fig. 2.   Fig. 2. Heating curve which is assumed in the heating system and its approximated polynomial.
Carbon dioxide emissions 2  CO during generation electricity consumed by a heat pump is estimated as follows: where: β -generation factor of a fuel (cf. S -share of the fuel in electrical energy generation (cf. Table 2).

The results and discussion
The results are affected by temperature of heat transfer medium. Since ground temperature is slightly lower than groundwater one, GSHP needs more work (electrical energy in the case) than WSHP to provide the same amount of heat. Hence, SCOP = 5.03 for GSHP is lower than SCOP = 6.12 for WSHP (cf. the paper [9]). Inasmuch as, SHE decreases brine temperature at 2°C in the WSHP circuit SCOP for WSHP+SHE is at 5.86 [9]. It is because each heat exchanger works at a temperature difference. After a substitution the above SCOP values the inequality (1) is verified. The results are plotted in Fig. 3. The relatively high magnitudes of SCOP causes each of analysed variant is ecologically viable as each heat pump would cause less CO2 emissions than condensing gas boiler. It differs from the conclusions in the paper [8], which results from lower COP values in that case. An advantage of the present analysis is caused by the application of hourly measured temperature values. Carbon dioxide emissions which are obtained from eq. (5) after a substitution the magnitudes from Table 2, are presented in Fig. 4. Because of the same reason the highest CO2 amount is emitted during GSHP usage. 18% less CO2 emissions are caused by WSHP usage. SHE instalation causes 3.7% higher CO2 production.

Concluding remarks
Temperature value at a heating system depends on a heating curve and the weather conditions, which are the same in all the investigated variants. Thus, it does not affect the results of the comparison. As, ground temperature is lower than groundwater temperature in the location, WSHP needs less energy delivered as work. Hence, WSHP causes lesser emissions in a power plant. Although, SHE installation generates a drop in temperature, risen CO2 emissions are lower than in the case of GSHP. After, a detailed analysis of carbon dioxide emissions in Białystok, we are allowed to conclude the ground-water or water-water heat pump can be applied as an ecological heat generator for the heating systems. Although, it is case study, the conclusions may be extended to less severe climatic zones in Poland. The particular decision which kind of heat pump should be applied, depends on a geological case and an economic analysis.