A thermo-economic and emissions analysis of different sanitary-water heating units embedded within 4 th generation district-heating systems

This paper presents the results of a thermo-economic, primary-energy-factor and CO 2 -equivalent (CO 2 (eq)), emissions-sensitivity analysis for the preparation of sanitary hot water (SHW) in 4th-generation district-heating systems. The annual required additional heat for the SHW provided by a local heating unit, based on an air-to-water heat pump, a natural-gas boiler and an electrical resistance heater, was determined using a TRNSYS simulation. Additionally, the seasonal performance factor of the heat pump under consideration was determined. The study considered three possible supply temperatures, i.e., 35, 40 and 45 °C. The results show that a local heating unit based on an air-to-water heat pump is the most efficient in terms of the used primary energy and CO 2 (eq) emissions. This unit is also the second best in terms of thermo-economic performance. The unit based on a natural-gas boiler is much more appropriate than an electrical resistance heater unit in terms of both the primary energy factor and the CO 2 (eq) emission factors for an electricity generation mix that has values higher than the average for the EU-28. The heat generated by this natural-gas unit is also cheaper than the heat produced by an electrical resistance heater based on the average price for electricity in the EU-28. This paper presents a TE, PEF and CO 2 (eq) EF analysis for sanitary hot water preparation in 4 th -generation district-heating systems. As a result of many positive effects on energy and exergy efficiency in the case of ultra-low DH supply temperatures, DH systems with supply temperatures down to 35 °C are currently under test. However, such low temperatures are not suitable for SHW preparation. This issue could be solved by applying a local heating unit, which could raise the SHW temperature to suitable levels. Based on a review of the currently available literature, there are no existing studies that would employ the TE, PEF and CO 2 (eq) EF analyses for local heating units that are based on a NG boiler, AWHP and ERH. The main novelty of the presented paper is the PEF and CO 2 (eq) EF analyses for the proposed local heating units. The PEF and CO 2 (eq) EF for the EGM and NG were determined by the GEMIS 4.95 model. Furthermore, this study has taken into consideration the real investment costs for the local heating units that are currently available on the market. All three analyses were performed as a sensitivity analysis, where the lowest, average and highest values were considered.


Introduction
District heating (DH) systems have been used all over the world for several decades because in conjunction with state-of-the-art combined heat and power (CHP) units it is possible to produce heat with a relatively low primary energy (PE) consumption and greenhouse-gas (GHG) emissions [1][2][3]. During this period DH systems have been divided into generations based on their size, efficiency, complexity and flexibility [4]. Currently, the 4 th generation of district heating (4GDH) systems has just been put into service [5].
Each generation brings a reduction in supply temperatures, i.e., for the 4GDH the supply temperatures are 50-60 °C [6]. In comparison, the supply temperatures of the 1 st generation DH systems, based on steam, were as high as 200 °C. Developments on going in DH systems could see even lower supply temperatures, with values as low as 35 °C being tested [7].
The main driver for lower flow temperatures is low-energy buildings. Lowering the supply temperatures has the following advantages: reduced heat losses through the pipes, utilization of waste-energy resources from industry, renewable energy, seasonal heat storage and surplus wind-based electricity [8][9][10][11]. However, the supply temperatures in 4GDH systems are not suitable for the preparation of sanitary hot water (SHW), which represents a challenge, particularly to sustain suitable temperatures of a SHW distribution. A possible solution to the required temperature increase of the SHW could be to combine local heating units with the 4GDH system [12]. There is, however, an open question about which heating technology is the most appropriate in such cases [12][13][14][15][16][17][18][19][20].
One of the first studies in which an exergetic evaluation of different heat pump (HP) booster configurations in a low-temperature DH was performed was reported by Ommen, T. S. et al. [13]. They suggested two main configurations: in the first one a HP was implemented on the primary side of the SHW heat exchanger, while in the second configuration a HP was implemented on the secondary side. They reported that the first case is superior to second case in terms of exergetic efficiency (0.52 vs. 0.44, respectively, at a supply temperature of around 45 °C).
Another study in which, besides exergy efficiency, energy efficiency and annual heating costs were also compared for several different heating systems for the supplementary heating of SHW, was performed by Elmegaard, B. et al. [14]. Those systems were based on conventional district heating systems, systems based on an electrical resistance heater and three HPs applying R134a and R744 refrigerants. The system based on a HP with the R134a refrigerant and with hot-water storage installed was reported to be the best in terms of exergetic efficiency (25%) among the systems based on HPs and an electric resistance heater. The overall best system in terms of exergetic efficiency (28%) was found to be a conventional low-temperature system. The same system also had the lowest annual costs for the heat production of sanitary hot water with €690 (compared to a system based on a HP for a 159 m 2 house with €740) and lowest CO2 emissions with a 1.0 Mg/year (compared to the system based on a HP with 1.1 Mg/year).
Despite the high exergy efficiency of low-temperature district heating systems, there is a great disadvantage of such systems, since these temperatures are suitable for Legionella proliferation with SHW circulation. To mitigate this problem, Yang X. et al. [15] proposed different systems with decentralized substations that could mitigate the aforementioned problem. The authors suggested that replacing a bypass pipe with an in-line supply pipe and a HP could reduce the heat losses by up to 39% compared to the conventional system and by up to 12% compared to the conventional system with a bypass. In a later study, Yang X. et al. [16] performed an evaluation of five different SHW heating systems in conjunction with ultra-low-temperature district heating. In order to avoid the proliferation of Legionella, supplementary heating devices were installed in all the substations. To compare all the systems, an energy and economic assessment was performed. They found that substations with in-line electric heaters had the lowest electricity consumption for SHW preparation (1.4 and 1.9 DKK/kWh), while the substations with storage tanks and HPs with a coefficient of performance (COP) of 4.5, had higher electricity consumption and, consequently, higher costs for SHW preparation (4.4 DKK/kWh).
A study in which energy, economic and exergetic performance for several different heating substations for three different supply temperatures of DH at 35 °C, 50 °C and 65 °C was performed by Yang X. et al. [17]. The COP of the heat pump that was considered in one example of a heating substation was 4.5. Similar to the previous study, they found that the substation with the in-line electric resistance heater performed better in the case of 50 and 65 °C supply temperatures, while in the case of the 35 °C supply temperature the best solution was reported to be based on an individual micro tank solution.
A feasibility study with a micro HP local heating unit (COP = 5.3) for SHW preparation in the case of a low-temperature DH system with a supply temperature of 40 °C was performed by Zvingilaite et al. [18]. Later, a similar study, but as a pilot experimental project, was performed by Brand, M. [19]. They investigated local heating units based on small HPs (also called micro boosters) with a COP of 4.5 in Birkerod (Copenhagen, Denmark). Each booster, with a 140 L tank, was installed in four houses built in the 1970s and used DH water as the source of heat. The increase in the temperature for the SHW was from 40 °C to 55 °C. This study revealed that the heat losses through the DH network in the case of low-temperature DH (40/25 °C) were reduced by 43%, compared to a conventional DH system (80/40 °C). Another interesting pilot project with local heating units based on an electrical resistance heater (ERH), also called an electrical booster, was performed in Odder (Denmark) [20]. The project involved five buildings, each having both floor and radiator heating. Each booster raised the temperature of the SHW from 37 °C to 45-60 °C. The heat losses through the DH network were reduced by 50% compared to a conventional DH system (80/40 °C). An interesting study in which the authors compared two types of SHW supply was performed by Østergaard P.A. et al. [12]. A DH system based on central HPs was compared to a DH system based on central HPs with an additional smaller booster HP. The main difference between the two systems was that the second system had the potential to lower the supply and return temperatures of the DH system. All the simulations were performed using the energyPRO [21] simulation model. The results show significantly lower DH temperature levels for the case of the DH system with an additional smaller booster HP. This leads to a higher coefficient of performance (COP) for the central HPs as well as to reduced heat losses through the DH pipes. They also found that this solution is better than an individual boiler or HP. However, the investment costs were not considered in this particular study as the authors reported that the micro booster technology is still under development.
A review of the literature shows that the combination of a low-temperature DH system with local heating units that raise the temperature of the SHW has great potential.
However, there are still some unresolved questions regarding the eligibility of a particular heating unit concerning thermo-economic (TE), primary energy factor (PEF) and CO2 (eq) emission factor (CO2 (eq) EF) analyses. The aim of this paper is to present the results of these analyses, in which several local heating units were compared. Those units, embedded in the 4GDH, are based on an ERH, an air-to-water heat pump (AWHP), and a natural-gas boiler (NG boiler). The main novelty of the presented paper is the PEF and CO2 (eq) EF analyses of the proposed local heating units by considering the PEF and CO2 (eq) EF for the electricity generation mix (EGM) and natural gas (NG) for the EU-28. Both factors were based on the GEMIS 4.95 model [22]. Furthermore, the particular study performed a thermo-economic (TE) analysis considering the electricity and NG prices, based on the EUROSTAT database [23] for the EU-28 in the period 2011-2015. All three analyses were performed as a sensitivity analysis, where the lowest, average and highest values were considered. In the TE analysis the investment costs of the local heating units that are currently available on the market were also taken into consideration.

Methods
The presented study was conducted for a four-family-member residential dwelling. In order to perform the TE, PEF and EF analyses in the case of different local heating units the required additional heat for the SHW was determined. In the case of a local heating unit based on an air-to-water heat pump (AWHP) the SPF was also determined. The required additional heat for both the SHW and the SPF were determined using the simulation tool TRNSYS. The schematics of the local heating unit used in a simulation with TRNSYS and its connection to the SHW storage, the heat exchanger, the DH and the sanitary cold water (SCW) supply are shown in Fig. 1.

Simulation of the required additional heat for the SHW
The following considerations were used in the simulation of the required additional heat for the SHW: • Analyzed period: 1 year (5-minute time step) • Each local heating unit was producing water at 55 °C. Due to heat losses (5 °C) the SHW at the user temperature was set to 50 °C • The heat-exchanger effectiveness was set to 0.95 • According to ISO 13612 the daily SHW consumption of a household with a family for shower use is 126 l/day of 50 °C water.
• According to ISO 13612 we assumed two different patterns of SHW usage for the weekdays and weekends, as shown in Fig. 3 • An anti-legionella measure was also considered. It was implemented each week for 1 hour by superheating the SHW to 62 °C • SHW storage is wrapped in heat insulation with a U value of 0.6 W/m 2 K and placed inside the dwelling.
• The SHW storage volume is 80 L and was chosen according to ISO 13612

Simulation of SPF for an AWHP
The following considerations were used in the simulation of the AWHP's SPF: • The evaporator of the AWHP was considered to be positioned outside the dwelling • The analyzed period was 1 year, with a 5-minute time step • The SPF calculation, which is the ratio between the generated heat and the consumed electrical energy of the AWHP, was performed using the test reference year of Ljubljana [24]. An example of a simulation-based calculation of an AWHP's COP is shown in Fig. 4.
• Frost formation on the evaporator coils in the case of an AWHP and a de-frosting program was not considered It is obvious that in the case of a water-to-water heat pump (WWHP) with an evaporator connected to the DH return, the SPF factor would be significantly higher. However, the micro booster WWHP technology is still under development, so the investment costs in the case of such a system are not known. There is also another option based on using the heat from the air inside the heating substation. However, various aspects influence the variation of the air temperature inside the heating substation, which is not a subject covered in this study. Based on this information we decided to use an AWHP that is currently available on the market with its evaporator placed outside the dwelling and with known investment costs. Additionally, we performed a simulation for two hypothetical cases in which we calculated the SPF factor of a WWHP that has an evaporator connected to the DH return and the SPF factor of an AWHP that has an evaporator inside the heating substation. All the parameters as well as the results of those hypothetical simulations are shown in Table 1. As expected, the SPF in both additional cases is significantly higher (WWHP SPF = 3.66 -4.00 and AWHP SPF = 3.60) than the SPF of the AWHP, where the heat source is the external air (AWHP SPF = 3.28).

Important simulation results
The main output of the proposed simulation was the annual consumption of additional required heat for the SHW, which was determined to be 1330, 1075 and 821 kWh/a in the case of DH supply temperatures of 35, 40 and 45 °C, respectively. This amount of heat includes the heat for the anti-legionella superheating program as well as the SHW storage heat losses. In the case of the SPF of an AWHP simulation the annual SPF factor, which was used in all subsequent analyses, was determined to be 3.28 for all three supply temperatures of the DH. The main reason for the SPF invariance with respect to the DH supply temperature is that the heat source and the heat sink of the AWHP were the same for all three DH supply temperatures. All the important simulation-based calculated values are presented in Table 2.

Thermo-economic assessment
The TE assessment, performed for three local heating units, based on a NG boiler, an AWHP and an ERH, considered the EU-28's electricity and NG prices for domestic consumers between 2011 and 2015. The prices for each individual EU country were found in the EUROSTAT database and include all taxes and levies. The TE calculation was based on the methodology that is thoroughly described in [25], but with the assumption that the additional annual electricity, cleaning and maintenance costs were not taken into consideration. The efficiency of the NG boiler was calculated according to the standard [26]. The main outputs of the TE calculation were the average final costs for heat production [€/kWh], the average annual final costs [€/a], and the relative share of final costs [/]. All the outputs also took into consideration the investment costs for all three local heating units that are currently available on the market. Therefore, the term final costs denotes the inclusion of the investment costs in the aforementioned outputs. The relative share of the final costs of the NG boiler and the ERH local heating units are calculated with respect to the AWHP. The main reason for that is the AWHP has the highest investment costs and to check whether or not the AWHP is competitive with the two local heating units that have significantly lower investment costs.
One example of the TE calculation by considering the EU-28's average prices for electricity and NG for domestic consumers with all three local heating units in the case of 1330 kWh/a for the required additional heat is shown in Table 3.

Assessment of the primary energy factor
A comparison of the primary energy factor (PEF) for the required additional heat generation between all three local heating units was based on the PEFs of the two heat sources: electricity and NG. The methodology for the PEF assessment used in this study is described in [25]. One of the main reasons why the PEF assessment is valuable is the inclusion of the primary energy required for extraction, transport, conversion, distribution, transmission and control losses of the particular heat/energy source [29].
The PEFs of the electricity and NG for the specific EU country in 2010 were provided by the GEMIS model database version 4.95 [22]. The average PEF value of the EGM (PEF = 2.48) in the EU-28 was also based on the GEMIS model and was calculated using PEF values between 2010 and 2013. Regarding the PEF of the EGM for a specific EU country, the PEFs were determined for all 28 EU countries (including the UK). This also applies to the PEFs of NG, but the data from the following countries were missing: Cyprus, Finland and Malta. In addition, the PEFs for both electricity and NG in Croatia were based on data from 2005.
An example of a PEF calculation that considers the average PEF for the EGM and NG for all three local heating units in the case of 1330 kWh/a of required additional heat are shown in Table 4.
Considering the lowest, average and highest PEFs for the EGM and NG, the AWHP is the most efficient in terms of the consumed PE for the whole range of the PEF for the EGM, exception is in the case of the highest PEF for NG, since this unit, with its relative share equal to 3.62, is outperformed by the ERH with a relative share of 3.28. This value is in fact the SPF factor for the AWHP.

CO2 (eq) emissions assessment
Numerous initiatives on a global scale are striving to reduce greenhouse-gas (GHG) emissions, which could help reduce the rates of global warming. Energy use, which corresponds to about two-thirds of human activities producing GHGs, is responsible for about 90% of the CO2 emissions [30]. Hence, besides the TE and PEF analyses, it is vital to assess the CO2 (eq) emissions for all three local heating units. The CO2 (eq) emission factor (CO2 (eq) EF) for heat production was calculated from: where ℎ denotes the CO2 (eq) emission factor for the heat source, denotes the efficiency of the boiler and SPF denotes the simulation-based seasonal performance factor of the AWHP. The annual emitted CO2 (eq) was calculated from: where denotes the annual required additional heat for SHW. The CO2 (eq) EFs for the electricity and natural gas of the specific EU country for 2010 were provided by the GEMIS model database version 4.95 [22]. The average CO2 (eq) EF value of the EGM (0.3538 kg CO2 (eq)/kWh) in the EU-28 was also based on the GEMIS model for the period 2010-2013. Regarding the CO2 (eq) EF of a specific EU country, the EFs were determined for all 28 EU countries (including the UK). This also applies to the EFs of natural gas, but the data from the following countries were missing: Cyprus, Finland and Malta. In addition, the EFs for both the electricity and NG in Croatia were based on data from 2005.
An example of the CO2 (eq) emissions calculation by considering the average CO2 (eq) EF for the EGM and NG of all three local heating units in the case of 1330 kWh/a of required additional heat are shown in Table 5.
The results of all the other examples for the lowest, average and highest values of the CO2 (eq) EF for the EGM and NG are shown in Fig. 9. The AWHP unit is the most efficient in terms of CO2 (eq) emissions across the whole range of EGM EFs, except for one case, as detailed below.
In the case of the heating unit based on the ERH the relative share of the emissions is 3.28 times greater than the AWHP across the whole range of EGM EFs. This unit is comparable to the unit based on a NG boiler, which has relative shares of emissions at 2.62, 2.94 and If the EGM EF approaches the highest value, the NG boiler has a smaller relative share of emissions (0.93) than the AWHP, but just in the case of the lowest NG EF. In the other two cases the NG boiler unit has a higher share (1.04 and 1.17) than the AWHP.

Conclusions
This paper presents a TE, PEF and CO2 (eq) EF analysis for sanitary hot water preparation in 4 th -generation district-heating systems. As a result of many positive effects on energy Hence, in this study we prepared a TRNSYS simulation in which the annual required additional heat for SHW was determined for a four-member-family residential dwelling. Considering the CO2 equivalent emissions analysis the unit based on the AWHP is again the most efficient in terms of CO2 (eq) emissions, except for the case of the EU-28's highest EGM emissions (1.000 kg CO2 (eq)/kWh) and the EU-28's lowest NG emissions (0.254 kg CO2 (eq)/kWh). In that case the unit based on the NG boiler has a 0.93 relative share compared to the AWHP. The ERH unit would emit 3.28 times more emissions than the AWHP. This relative share is comparable to the share of a unit based on a NG boiler (2.62-3.30) in the case of the average EGM EF (0.354 kg of CO2 (eq)/kWh). The NG boiler unit has the highest relative share (9.64-12.15), which is seen in the case of the lowest EGM EF (0.096 kg CO2 (eq)/kWh).
The final highlights based on the presented results are: • The local heating unit based on the AWHP is the most efficient in terms of the consumed PE and CO2 (eq) emissions. Because of the relatively high investment costs, this unit is the second best in terms of thermo-economics across the whole range of the EU-28's prices for electricity. However, this study considered an AWHP where the evaporator is outside the dwelling. We also performed a hypothetical study in which the SPF of an AWHP and a WWHP unit were determined. In the case of both units, the evaporator was placed inside the heating stations and connected to the DH system's return. The calculated SPFs were in the range 3.60-4.00, which could, based on the AWHP, be even more attractive with respect to the TE.
• In some EU markets with the lowest electricity prices the units based on ERH possess a great threat to technologies that are significantly more suitable in terms of environmental aspects. We proved that in the case of the EU-28's lowest electricity price the ERH unit has a relative share of final costs equal to 0.66-0.94, compared to the AWHP. This was also proven for the case of the Slovenian market, since the price ratio between electricity and NG (1.3) is much lower than the EU-28's average (2.05).
• The unit based on the NG boiler is significantly more appropriate than the ERH unit in the case of PEF and CO2 (eq) EF for EGM values higher than the EU-28's average. From the TE point of view the NB boiler unit is also cheaper in the case of the average and the highest electricity prices in the EU-28.
• In EU markets with the lowest CO2 (eq) EF for EGM the use of NG boiler units must be seriously considered, since compared to the AWHP a NG boiler would emit approximately 10 times more CO2 (eq).