Energetic and economic analysis of decoupled strategy for heating and cooling production with CO2 booster heat pumps for ultra-low temperature district network

Depósito de Investigación de la Universidad de Sevilla https://idus.us.es/ This is an Accepted Manuscript of an article published by Elsevier in Journal of Building Engineering, Vol. 45, on January 2022, available at: https://doi.org/10.1016/j.jobe.2021.103538 © 2021 Elsevier. En idUS Licencia Creative Commons CC BY-NC-ND Energetic and economic analysis of decoupled strategy for heating and cooling production with CO2 booster heat pumps for ultra-low temperature district network Gonzalo Quirosaa*, Miguel Torresa, Víctor Solterob, Ricardo Chacarteguia aDepartment of Energy Engineering. Universidad de Sevilla. Escuela Técnica Superior de Ingenieros. Avenida de los Descubrimientos, s/n, 41092 Seville, Spain bDepartment of Design Engineering, Universidad de Sevilla. Escuela Politécnica Superior. Calle Virgen de África, 7, 41011 Seville, Spain * Corresponding author Abstract The ultra-low temperature district heating and cooling (ULTDHC) systems are among the most interesting solutions for the decarbonisation of heating and cooling sectors. It is a technology where different optimised integrations and operations are still under development. The booster heat pumps (BHPs) or substations associated with this new generation take on a more critical role because the energy supplied by the main network is not enough for covering the user's requirements. Reviewing the literature, it has been seen necessary to make a short and long-term energetic and economic analysis of BHPs operation (considering heating, cooling and SHW requirements) under variable ULTDHC conditions. On this line, this work focuses on the analysis of the operation of two of the most promising BHP types: one with a single CO2 heat pump, type 1, and the other with two CO2 heat pumps with a decoupled operation, type 2. Both BHPs are connected to an ultra-low temperature District Heating and Cooling network with variable parameters throughout the year. The simulation is carried out in a representative building located in Toledo (Spain), in an area with considerable heating and cooling demands throughout the year. The objectives of the work are the following: analyse the daily and annual operations of the two BHPs types, study the effect of outdoor temperature on their optimal operation, point out their main parameter influence and variation and make an economic evaluation of these systems. The BHP type 2 investment is 8.3% more expensive than type 1 but the economic analysis shows an annual saving in heating and cooling operation of the building of 7.7% for BHP type 2 regarding type 1. The size of the BHP, total demand and buildings supplied affects the profitability of the BHP type. For this analysis, type 2 shows more profitable than type 1 when the annual heating demand is greater than a certain threshold. Keywords Ultra-low temperature district heating and cooling, 5th generation district heating and cooling, CO2 booster heat pump, Substation, Energy transfer station 1. Introduction The new district heating and cooling networks called 5th Generation District Heating and Cooling (5GDHC) or Ultra Low-Temperature District Heating and Cooling (ULTDHC), which operate at temperatures around 20-40 °C, are considered as a viable technology to satisfy thermal requirements in a sustainable way [1]. Although District Heating and Cooling is a very mature technology, in-depth research is still being carried out to adapt its operation to the current requirements of the energy sector so that distribution losses are minimal and lowtemperature renewable sources can be integrated [2]. Besides, they also allow the successful integration of PV and wind for heating and cooling through substations heat pumps. The evolution of these systems has always been aimed at reducing supply temperatures as far as possible concerning previous generations to increase energy efficiency [3]. It has allowed, for example, DHC networks to be implemented cost-effectively in rural areas with low demand density values [4]. With the decreasing space heating and cooling demands of new buildings due to the increase of energy efficiency requirements in buildings, energy losses in distribution become more important (relatively), as less energy needs to be transported until the end-user [5]. Low-temperature district heating (4th generation district heating, 4GDH) has the same objective as ULTDHC, which is to improve the decarbonisation of the thermal energy sector. ULTDHC is focused on combining heating and cooling requirements efficiently with reduced temperature networks, while currently, 4GDH is usually more competitive when only there is heating demand. In this sense, ULTDHC should not be seen as an evolution of 4GDH but as complementary options regarding design and integration[6]. Network operating at low temperature provides a significant increase in versatility for coupling with other energy sources and systems, broadening the range of applications to be integrated within the DHC. They allow the simultaneous supply of heating and cooling from the warm water with which the network operates [7]. Therefore, integration of buildings in a 5GHDC network or in which cases it is interesting to adapt the existing old network to the concepts of this new generation [8]. An interesting aspect is that the ULTDHC distribution network can act as a shared thermal storage system for all the users involved due to the low level of energy losses. As the temperature drop in the substations is smaller than previous DH designs, they require an increase of the circulating flow, an increase in the diameters of the distribution pipes, and hydraulic pumping costs [2,9]. On the other hand, to compete cost-effectively with Low- Temperature District Heating (LTDH) systems, the cost of the decentralised units can be reduced, the investment of the generation plant is considerably lower in the case of ULTDH, and they can use a broader range of energy sources [10]. In this new generation of DHC, Energy Transfer Station (ETS) substations gain importance as they are responsible for boosting the energy and adapting the working fluid to the levels required by the building's internal installations. It means that they have higher costs than previous generations of DHCs. The basic version of the substation in ULTDHC usually consists of using a heat pump and a thermal storage system for DHW, and the correct control of these is key to increase the potential and efficiency of the whole system [11]. To increase the potential, substations must also be designed to take advantage of surplus distributed renewable generation and feed the energy into the distribution grid and have different operating modes depending on the situation at any given moment [12]. Substations can also add exchangers to recover low-temperature waste heat sources in the cities [13,14] (metro stations, sewage networks, transformer stations) that can be cost-effectively integrated within ULTDHC networks. An important aspect is the return temperature to the distribution network, as the overall DHC efficiency depends on this value, and it varies depending on the operation conditions, for example, when producing Sanitary Hot Water (SHW) [15] or when there are low demand periods. It is common to use bypasses to keep the return pipes at an acceptable temperature, but more efficient solutions can be achieved [16,17]. In this framework of ULTDHC development, the study of substations or booster heat pumps (BHPs) as components that integrate the building with the network and their optimised operation is essential for their successful development. In this line, there are comparative papers of great interest [18,19], and it is a field of study in which the optimisation of the systems is modified depending on the type of user, environmental characteristics and availability of energy sources. It is an open research field where there are opportunities to further contribute to new concepts and analyses to develop these networks further. The current state of the art focuses on BHPs for LTDH networks, although it is possible to adapt some of these typologies to the case of the new generation. For example, there has been considerable research on how BHPs can harness geothermal energy for system integration [20]. Now, ULTDH BHPs can utilise low-temperature geothermal sources that were not possible before, increasing the applicability options. One of the most promising technologies for achieving energy sustainability since it can be used as a bridge between the electricity and thermal sectors is the use of heat pumps in District Heating and Cooling, so their research and development in terms of operating modes, connection and location within these systems is key [21] addition, they are capable of alternately generating heat and cooling, making their integration in tri-generation systems perfectly viable. Simultaneously, they are sufficiently flexible in operation and size to be used in the main DHC systems and support or backup systems [22]. In [23] is shown that the use of decentralised heat pumps together with insulated thermal storage systems in low-temperature DHCs leads to the best results in terms of performance, compared to the most popularly known and implemented DHC systems. It is also important to note that heat pumps are a great option to achieve greater integration of distributed renewable energy sources due to their flexibility of operation. However, in economic analysis, good results are not always obtained, as they are often oversized to gain in reliability, which makes them unprofitable [24]. Heat pumps are also effective when establishing models focused on responsible demand so that DHC demand peaks can be smoothed, thereby increasing efficiency and reducing costs [25]. Focusing on other papers related to BHPs in ULTDHC has been found the following. In [26], an interesting study analysing the steady-state thermal behaviour of Booster Heat Pump with different temperatures and flow rates is carried out but no economic analysis or long term operation has been made. In [27], five types of substations for SHW production were studied but it is not considerer the heating and cooling requirements. A novel system based on heat pumps is analysed in [28] and applied to a real case study, showing interesting results. Nevertheless, in this paper, the aim is to evaluate two of the most common type of BHPs so that the results obtained can be applied to current situations. In [29] a good analysis is carried out regarding heat pump efficiency in networks using a wastewater heat source but the network supply temperature is constant in the study and this is not the real situation considering a whole year of operation. A hybrid model predictive control strategy is proposed and implemented in a real location in [30] with interesting results, although no energy or economic analysis is done. The BHPs associated with this new generation take on a more critical role than in previous generations. Reviewing the literature, a gap related to the energetic and economic analysis of common BHPs types (considering heating, cooling and SHW requirements) under variable ULTDHC conditions has been found. Moreover, few studies of long-term operation have been found for ultra-low temperatures. It is necessary to study their performance because their operations are different from those in the previous generations. They work with lower temperatures supply, they need to increase and adapt the energy and they cover heating and cooling demand simultaneously. Moreover, the influence of main parameters in their performance, like the level of supply temperature, is not quantified. This article aims to deepen the knowledge of ULTDHC substations, emphasising two of the most interesting typologies. The heat pumps, which will be the main elements, will be distributed differently, and their operating modes will vary to obtain a complete analysis. An annual analysis of their operation under ULTDHC conditions, an evaluation of the outdoor temperature influence, a study of their main parameters and an economic evaluation will be the main results of the paper. The results will allow conclusions drawn for these typologies. The simulations will provide insight into their operation over their entire life cycle, analysing the influence of the outdoor temperature on the different operating modes. In this way, the aim is to cover and advance the study of the systems that act as a connection between the ultra-low temperature networks and the users' installations to meet their requirements efficiently. Therefore, it is important to take into count that all the results obtained for the BHPs do not come from studying them in isolation but considering a system including the ULTDHC main network. The results obtained are valid for booster heat pumps or substations in ULTDHC conditions so the work is quite specific and original. Although, the study allows to reach conclusions for other types of situations as well. 2. Material and Methods 2.1. Selection of the analysis location For the analysis, a location with representative heating demand in winter and cooling demand in summer is considered. This location has been selected as representative of one in which traditional DH hardly could be cost-efficiently integrated because of the reduced heating demand, but the combined demand could be of interest for integrating a ULTDHC. It will allow analysing the integration of different substations and operation modes to cover both heating and cooling and SHW demand. It is located in Toledo (Spain), in the South part of the continental region of Spain. Fig. 1 shows the ULTDHC energy production plant and covered users considered for the case study. The coordinates of the location are 39.883, -4.0318 (Decimal Degrees). The average temperature and the total solar irradiation reached on each day of the year are shown in Fig. 2. Fig. 1 Location of the ULTDHC energy production plant and users Fig. 2 Daily temperature and irradiation The average temperature of each day varies from 29.8 °C to -0.95 °C. The annual average temperature in the location is 13.6 °C. Regarding direct irradiation, 8.81 kWh/m2, 0 kWh/m2 and 2.72 kWh/m2 are the maximum, minimum and average daily values respectively. For indirect irradiation, these values are 3.10 kWh/m2, 0.14 kWh/m2 and 1.52 kWh/m2. The total annual irradiation in the zone is 993.6 kWh/m2 for direct and 555.2 kWh/m2 for indirect. The combined demand of this location would allow deriving conclusions on the performance of the systems and BHPs and extrapolate results to other climate zones with this mixed operation, thus obtaining a complete study. 2.2. Energy Characterization The BHPs will be connected to a representative building located in Toledo to study their different operation modes and performance. In this way, it will be possible to analyse their behaviour in detail without interference from other users. From the climatic conditions and the characteristics of the typical dwelling in this zone [31,32], the heating, cooling and SHW requirements demanded throughout the year can be obtained. On the other hand, the building will have a renewable thermal self-consumption system based on solar collectors. They are mandatory by Spanish regulation on new buildings since 2006, when it was published [33]. It allows analysing the influence of distributed generation in the BHPs under study and in the ULTDHC network, which allows these technologies to be more easily integrated as they operate at lower temperatures. The characteristics and demands of the representative building can be seen in Table 1. Heating Degree Days (HDD) and Cooling Degree Days (CDD) patterns are presented in Fig. 3. The reference temperatures for the calculation of HDD and CDD were 17 °C and 25 °C, respectively. The maximum daily values for HDD and CDD are 17.9 °C and 4.8 °C respectively. Moreover, the annual accumulated HDD is 1948 °C and for CDD is 71.3 °C. It can be seen that the heating demand is much greater than the cooling one, due to the climatic evolution of the location. Table 1 Representative block of flats characteristics and demand Specification Number of dwellings 12 Surface per dwelling (m2) 85 Numbers of inhabitants per dwelling 2.5 Space heating power (kW) 65 Space cooling power (kW) 45 Equipment intermittency coefficient 0.62 Solar collector surface (m2) 24 SHW annual energy demand (MWh) 19 Space heating annual demand (MWh) 174 Space cooling annual demand (MWh) 20 Solar collector annual generation (MWh) 15 Fig. 3 Annual evolution of heating and cooling degree days in the selected location. For SHW demand, it was identified three peaks of demand at 10 am, 7 pm and 8 pm. Normal probability distribution has been used to simulate the SHW user demand during the day. 2.3. ULTDHC reference module characterisation For this analysis, the BHP and the building being supplied are part of a ULTDHC network that supplies a more significant number of users. The most important parameters for the operation of the BHP are the flow temperature of the ULTDHC and the water flow rate of the supply pipe to a dwelling. This flow rate is considered to have a maximum value of 1.4 l/s [8,34]. The supply temperature of the DH is considered to fluctuate with the ambient temperature of the area in order to minimise energy losses in the distribution. This supply temperature strategy can be implemented because direct-cooling systems will not be available in the BHPs [35]. Another possible strategy that could have been considered is free-floating, but it has been discarded because they do not use optimised temperatures, although they may be of interest in some situations. In the case study, distribution network temperatures will vary throughout the year between 20 °C and 30 °C. 2.4. Booster heat pumps scenarios Two different scenarios were considered, with a different type of BHP in each of them. They were selected based on the most common typologies used in the current literature related to ultra-low temperature networks. It has also been taken into account that they are systems applicable to typical residential areas without specific characteristics, such as the availability of other energy sources (i.e. geothermal or waste heat). Both typologies are designed to allow users to act as prosumers of energy to realise the full potential of the ULTDHC grid and distributed generation systems. The main element of the substation is a heat pump, which will allow it to operate in heating or cooling mode so that there can be users on the grid operating in different ways simultaneously. The BHPs are operating connected to a ULTDHC system, so the conditions of this network impact and determine the operation of the BHPs. The components with the greatest impact are heat pumps as their power, consumption and performance are determined by the temperatures and flow rates of the ULTDHC network. These temperatures and flow rates vary throughout the hours and days depending mainly on the level of demand and the climatic conditions. For that reason, the final results of the paper are related also to the performance of the main ULTDHC network. The different scenarios proposed are set out below. 2.4.1. Booster heat pump 1: Simultaneous Heat Pump The first type of BHP is the most usual configuration in real ULTDHC installations. This type of installation aims to satisfy all requirements in the simplest, most direct way and with a reduced investment. A single simultaneous heat pump covers heating and cooling demands. The basic scheme can be seen in Fig. 4, where the flows are differentiated according to whether the mode of operation is heating (winter season) or cooling (summer season). The operation of the BHPs is explained below. Fig. 7 Booster heat pump 2 simulation process flowchart The controllers for the valves, variable speed hydraulic pump and heat pump are programmed to act logically based on key readings such as the average temperature of the energy storage system (ESS), air temperature in the space heating and cooling zone, solar collector water outlet temperatures, flow rate and temperatures in the pipes and the operating mode of the heat pump. Depending on the values taken by the controllers, the diverters opening will vary and the flow will be supplied for specific pipes as well as the heat pump power and hydraulic pump power will be adapted. With the control algorithm, the system will work properly and with controlled and safe values. After the simulation has been performed, the results are for each minute of the simulation so they need to be processed to obtain global results. It is important also for economic analysis as they are inputs of this process. The sensitivity analysis has been done taking into account the following. Each BHP type has been simulated under different stable conditions, depending on the outdoor temperature value, during a day. For each outdoor temperature, the demand profile for users can be obtained, as well the as temperature levels and flow rates of the ULTDHC network. Considering these conditions, the BHPs are simulated and energetic results for their operation under this stable and particular outdoor temperature can be calculated. It is possible to obtain values like average COP, return temperature or energy consumed by the heat pump. With the sensitivity analysis is possible to obtain annual global results for the BHPs as their operation for each outdoor temperature situation is known and can be extrapolated regarding the outdoor temperature profile for a year. On other hand, the component programmes (simulated elements) have been created based on the following assumptions and equations: Water-to-water heat pumps are the main elements of the substations. The most influential temperatures on pump performance are the inlet water temperature on both sides of the system and the flow or outlet temperature of the target fluid. In this study, a water-to-water heat pump from the manufacturer Tecna, model 150W, was used [44]. For the heating mode, the coefficient of performance (COP) is used, the ratio between the heat flow in the condenser and the electrical energy consumed by the compressor, eq. 1, while in the cooling mode energy efficiency ratio (EER) is used, where the useful energy is the energy exchanged in the evaporator, eq. 2. 𝐶𝑂𝑃 = 𝑄󰇗𝑐𝑑 𝑊󰇗𝑐𝑜𝑚𝑝 ( 1) 𝐸𝐸𝑅 =𝑄󰇗𝑒𝑣 𝑊󰇗𝑐𝑜𝑚𝑝 ( 2) Where 𝑄󰇗𝑐𝑑, and 𝑄󰇗𝑒𝑣 are the heat exchanged at the condenser and evaporator, respectively and 𝑊󰇗𝑐𝑜𝑚𝑝 is power consumption in the compressor. On the other hand, by considering the energy balance of the heat pump, the eq. (3) is obtained, which allows the calculation of the rest of the parameters [41]: 𝑄󰇗𝑐𝑑 =𝑄󰇗𝑒𝑣 +𝜂𝑒𝑙𝑊󰇗𝑒𝑙,𝑐 ( 3) Where 𝑄󰇗𝑐𝑑 is the condenser thermal power, 𝑄󰇗𝑒𝑣 is the evaporator thermal power, 𝜂𝑒𝑙 the compressor efficiency and 𝑊󰇗𝑒𝑙,𝑐 is the compressor electrical power. The thermal storage system is also a key element, and the losses that occur are derived from the heat transfer expressions and can be obtained from eq. (4). 𝑄󰇗𝐸𝑆𝑆 =𝑈𝐸𝑆𝑆𝐴𝐸𝐸𝑆∆𝑇 ( 4) Where 𝑄󰇗𝐸𝑆𝑆 is the instantaneous thermal power of heat losses in the ESS, 𝑈𝐸𝑆𝑆 is the overall heat transfer coefficient of the ESS, which depends mainly on the materials used as insulation, 𝐴𝐸𝐸𝑆 is the heat transfer area of the ESS and ∆𝑇 is the temperature difference between the fluid stored inside the ESS and the outside temperature. To quantify the energy generated by solar collectors, eq. (5) [45,46] can be used. The commercial solar collector selected for the study is the model S21, Silver Family, Termicol Catalogue [47]. 𝑄𝑢=𝑄𝑇−𝑄𝑙𝑟 =𝐴𝑎𝑏𝑠𝐹𝑟[𝐺𝜏𝛼−𝜀𝜎(𝑇𝑤 4−𝑇𝑎𝑚𝑏 4)]=𝐴𝑎𝑏𝑠𝐹𝑟[𝐺𝜏𝛼−𝑈𝐿(𝑇𝑤−𝑇𝑎𝑚𝑏)] ( 5) Where Qu is the effective heat absorbed by the working fluid, 𝑄𝑇 is the total solar energy received, 𝑄𝑙𝑟 is the heat lost by radiation, 𝐴𝑎𝑏𝑠 is the energy absorption area that can be considered as half of the total internal heat transfer area of the tubes, 𝐹𝑟 is the heat removal factor, 𝐺 is the intensity of direct solar radiation, 𝜏 is the transmittance of the glass, 𝛼 is the absorptivity of the selective layer (𝜏𝛼 is the optical efficiency of the collector), 𝑈𝐿 is the overall loss coefficient, 𝑇𝑤 is the average collector temperature, is 𝑇𝑎𝑚𝑏 the ambient temperature, 𝜎 is the Stefan-Boltzmann constant (5.67x10-8W/m2K4) and 𝜀 is the emissivity of the selective layer. For the heat exchangers, it has been assumed that they are correctly designed and installed and operate annually around a constant efficiency value. Based on this assumption, equation (6) can be used for the analysis. 𝑄󰇗ℎ𝑒 =𝜀𝑄󰇗𝑚𝑎𝑥 =𝜀𝐶𝑚𝑖𝑛∆𝑇𝑚𝑎𝑥 =𝜀𝐶𝑚𝑖𝑛(𝑇ℎ,𝑖𝑛 −𝑇𝑐,𝑖𝑛) ( 6) Where 𝑄󰇗ℎ𝑒 is the instantaneous operating power of the heat exchanger, ε is the efficiency, 𝐶𝑚𝑖𝑛 is the mass heat (𝐶 =𝑚𝑐𝑝), 𝑇ℎ,𝑖𝑛 is the inlet temperature of the hot fluid exchanger and 𝑇𝑐,𝑖𝑛 is the inlet temperature of the cold fluid exchanger. The power of the hydraulic pump, 𝑃ℎ𝑝, is calculated by the equation (7), where 𝑄 is the flow rate of the pump, ϒ is the specific gravity, ℎ is the head of the pump and 𝜂ℎ𝑝 is the pump efficiency. 𝑃ℎ𝑝 =𝑄ϒℎ 𝜂ℎ𝑝 ( 7) By correctly assembling all these expressions, it will be possible to evaluate the temperatures reached at reference points of the installation, their mass flows and instantaneous powers at each operating instant, taking into account the different operating modes. These results will allow a complete technical analysis of the two types of BHPs with a short time horizon (daily analysis) and a long time horizon (annual analysis). Another important point is the consideration of the massiveness and the thermal inertia of the buildings. A building with a large thermal mass can level out the fluctuations in thermal energy flow, so the non-consideration of this phenomenon can imply important errors in short and long-term simulations [48]. The simulation process of this aspect has been carried out following the model in [49]. On the other hand, the values considered in the study for the buildings thermal characterisation come from [50]. The model validation has been done in various steps. In the first step, the simulation has been analysed step by step in the critical parts of the simulation. Then, the results have been studied and verified to be logical and reasonable. In this step, the input data have been modified in different simulations to check that the results or outputs vary reasonably. Finally, the simulation has been carried out with case studies easy to estimate their final values, with the result that the model fits well as expected. 2.6. Economic analysis modelling In this section, it will be interesting to carry out an economic analysis of each of the proposed scenarios. The economic study will consist of calculating the annualised costs of both proposals and comparing them. It will also calculate the break-even point of interest of carrying out each of the proposals, which will depend mainly on the annual demand for heating and cooling to be covered by the system. For the economic analysis is important to know the behaviour of the BHPs throughout a year of operation so a long-term energetic analysis has to be carried out. It is necessary to obtain annual values for ULTDHC energy consumption, energy consumed by the heat pumps, hours of operation and energy generated by the solar collector. It will be necessary to differentiate and calculate: CAPEX (capital expenditures): Investment required to carry out the installation. Included are the costs of components, materials, construction, installation, commissioning and taxes. 𝑂𝑃𝐸𝑋 (operational expenditures): operating costs consisting of the contribution of several items: fuel cost, electricity, maintenance, taxes, equipment and plot rental among others. A large part of these costs depends directly on the thermal demand. It has been calculated from Equation (7) [51] using the costs of operation and maintenance of the first year, 𝐶𝑂&𝑀,𝑦1, the costs of energy to be purchased, 𝐶𝑒𝑛𝑒𝑟𝑔𝑦,𝑦1, considering the service life of the installation, 𝐿𝐶, and the annual increase in prices 𝑃𝐼. Equation (8) shows the composition of the energy costs, consisting of the sum of the energy costs of the DHC, CDHC, and of the electricity required by the heat pumps, Celectr. 𝑂𝑃𝐸𝑋 = ∑[𝐶𝑂&𝑀,𝑦1 +𝐶𝑒𝑛𝑒𝑟𝑔𝑦,𝑦1]·[1+𝑃𝐼(𝑦−1)] 𝐿𝐶 𝑦=1 ( 8) 𝐶𝑒𝑛𝑒𝑟𝑔𝑦 = 𝐶𝐷𝐻𝐶 +𝐶𝑒𝑙𝑒𝑐𝑡𝑟 ( 9) 𝐴𝐶 (annualised costs): shall consist of the sum of the necessary investment, CAPEX , plus the annual operating costs, OPEX , over the total years of useful life (life cycle), LC , of the system. Equation (9) has been used. 𝐴𝐶 =𝐶𝐴𝑃𝐸𝑋+𝑂𝑃𝐸𝑋 𝐿𝐶 ( 10) The economic values used as a reference in the study, both for equipment costs and energy costs, are shown in Table 2. It should be taken into account that certain systems such as the solar collector, ESS or medium temperature radiators are considered to be already installed in the dwelling and do not influence the initial investment. For BHP type 1, it has been considered that the dwelling does not have low-temperature systems installed for space heating, so it is necessary to include radiators for this purpose in the investment. Table 2 Data for Economic Analysis Price Reference Water-to-water heat pump (€/kW) 360 [41] Installation cost BHP 1(€) 20000 [52,53] Installation cost BHP 2(€) 22000 [52,53] Maintenance cost (€/year) 3 % of CAPEX [41] Electricity price (€/kWh) 0.1128 [54] ULTDHC energy price (€/MWh) 10 [41] Service Life (years) 30 [7] Annual Price Increase 1 % [51] 3. Results 3.1. Technical Results The results obtained are shown below. First, the operation of the BHPs is studied on a typical winter day to analyse the different operation modes. The simulations have been carried out minute by minute throughout the day so that those elements that act in a very punctual manner, such as those used for SHW production, can be seen correctly. Fig. 8 shows the temperature and solar radiation profile for a typical operating day in winter and summer. In winter, the maximum and minimum temperature values are close, so the variation in the demand profile will not be substantial, as is the case for temperatures. On the other hand, the maximum daily radiation value for this reference day is around 194 W/m2. In the case of the summer day, the temperature profile variation is more significant, and the maximum value of radiation is higher, 838 W/m2. Fig. 8 Ambient temperature and radiation evolution in reference days Figure 6 shows the most relevant results of the daily operation in the winter of BHP 1. It shows the COP and EER, operation power and temperatures reached at different points. Fig. 9 BHP 1 operation on a winter day The COP varies considerably depending on the mode of operation, reaching values throughout the day between 4.7 and 3. The COP value depends mainly on the conditions between which the heat pump is working. Among other things, it will be higher when the evaporator and condenser temperatures are closer together. Therefore, the higher the DHC water inlet values and the lower the heat pump (HP) supply flow temperature values, the better the COP values will be. For this reason, when the system requires raising the ESS temperature, the return temperature values in the system increase, and the COP value decreases. The heat pump compressor (HPC) power over the minutes of the day shows that the power peaks are produced when the ESS needs to be charged when its average temperature is below 60 °C. It coincides with SHW consumption peaks by the building users or when the tank is discharged, as at the beginning of the day. The maximum value for a representative winter day is 20.5 kW. On the other hand, it can be seen the instantaneous power consumption of the DHC energy to cover the heating requirements. It is linked to the demand in the building and has a similar profile to the compressor power curve but maximised as a function of the DHC supply temperature and the COP of the HP. For the presented winter day, there is a maximum value of 44 kW. The heat pump in this type of BHP works with an approximately constant supply temperature of 65 °C. In the space heating radiators, a significant temperature drop, with the water temperature fluctuating around 20 °C. This value depends on the ambient temperature of the area to be heated. Finally, it can be seen the charge and discharge cycles of the ESS. Its average temperature decreases due to energy losses and SHW consumption. When the charging process is activated, the flow in the ESS branches is activated and water from inside the ESS circulates in the return branch, waiting for the HP to increase its temperature. In Fig. 10 it can be appreciated the operation of BHP 2 on a typical winter day. Fig. 10 BHP 2 operation on a winter day The COP of the HP for space heating increases considerably compared to the previous case, as it works under more optimal operating conditions and because the supply temperature of the SHC HP is lower, reaching a daily value of around 5.6. However, in the HP case for DHW, the opposite is true, as it is only used to cover occasional high demand requirements, reaching a value of over three during operation. Compared to the results obtained in BHP 1, it can be seen that there are now two independent HP in operation. The HP used to cover the space heating requirements now has a very stable operation throughout the day, as fluctuations due to ESS demands are taken care of by the SHW HP. This means that the heat pumps to be used may have a lower design power and are more specific and more optimised. The maximum running power of the compressor for the space heating and cooling (SHC HPC) is 6.7 kW, and for the sanitary hot water heat pump compressor (SHW HPC) is 11.7 kW. As for the profile of the instantaneous power required of the DHC network, it is very similar to that of BHP 1, as the demands of the two heat pumps are coupled. This means that from the DHC point of view, these two types of BHPs do not have noticeable differences. There are two different operating temperatures, around 35°C for the low-temperature radiators and the other 65°C for the ESS charging process. The thermal jump that now occurs in the radiators is considerably lower than in the previous situation. In the SHC branch, the energy losses that occur in this case are lower because it operates with a lower flow temperature. The operation of both BHPs is studied on a typical summer day. Fig. 11 shows the operation of BHP 1. Fig. 11 BHP 1 operation on a summer day First of all, it is important to note that when the pump operates simultaneously, the compressor can cover the heating and cooling requirements simultaneously too, using the condenser and evaporator, respectively. In this way, it is possible to balance part of the system and reduce the required consumption of the DHC. In this mode, COP and EER are differentiated. The COP is related to the pump operating in heat mode, with a value around 3. The EER is calculated when only cooling demand is present, with approximately constant values of 4 during the operating hours. Therefore, in this case, the COP is related to the SHW production and the EER with the cooling demand. In terms of compressor power, a distinction is made when the compressor is operating to obtain heat energy (SHW requirements) or cold energy (space cooling requirements). When the HP works simultaneously, the compressor is considered to be in heat mode and consumes from the DHC only the energy required to satisfy the energy balance, considering the current decrease due to the cooling requirement of the installation itself. The refrigeration requirements are given mainly in the central hours of the day with higher ambient temperatures. The maximum peak compressor power for cooling is 12.6 kW and for heating 11.4 kW. In this case, it is consumed from the ULTDHC network for both heating and cooling requirements. Therefore, sometimes the water temperature discharged into the network will be greater than the supply (cooling), and at other times it will be less (heating). The maximum peak power of the DHC for cooling requirements is 64.2 kW and 12.4 kW for heating. The HP supply temperature for cooling is 15 °C, and the temperature drop produced in space cooling varies depending on the ambient temperature in the area. With higher the temperatures, the higher is the refrigeration requirement. The energy losses that occur in the ESS are lower because of the smaller difference of temperatures between the ESS and the ambient temperature of installation. Fig. 12 shows the operation of BHP 2 on a typical summer day. Fig. 12 BHP 2 operation on a summer day In this case, although they are two isolated pumps, the resource water pipes are linked, so that their requirements can be compensated when both are working at the same time. This is because one of them will require heat at the source for its operation while the other will need cooling. This situation will result in less energy being consumed by the DHC. In this case, it can be seen that both the instantaneous powers throughout the day and the temperatures at the different points of the installation are pretty similar to the results of BHP 1, except for the fact that the parameters of the heat pumps are represented separately. 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