Analysis of a combined heating and cooling system model under different operating strategies

. The paper presents an analysis of a combined heating and cooling system model under different operating strategies. Cooling demand for air conditioning purposes has grown steadily in Poland since the early 1990s. The main clients are large office buildings and shopping malls in downtown locations. Increased demand for heat in the summer would mitigate a number of problems regarding District Heating System (DHS) operation at minimum power, affecting the average annual price of heat (in summertime the share of costs related to transport losses is a strong cost factor). In the paper, computer simulations were performed for different supply network water temperature, assuming as input, real changes in the parameters of the DHS (heat demand, flow rates, etc.). On the basis of calculations and taking into account investment costs of the Absorption Refrigeration System (ARS) and the Thermal Energy Storage (TES) system, an optimal capacity of the TES system was proposed to ensure smooth and efficient operation of the District Heating Plant (DHP). Application of ARS with the TES system in the DHS in question increases net profit by 19.4%, reducing the cooling price for consumers by 40%.


Introduction
Cooling demand for Air Conditioning (AC) purposes has grown steadily in Poland since the early 1990s. The main clients are large office buildings and shopping malls located in city centers. In widespread conventional cooling systems, demand for air conditioning is met by centrally installed compressor chillers in Compressor Refrigeration System (CRS) producing chilled water streamed to receiversfancoils or coolers in air handling units, using a dedicated hydraulic installation inside the building. Electricity is the driving medium for producing chilled water in CRS. There is an increasing scarcity and cost associated with the medium, especially in light of recent summertime blackouts in Poland. There is a steady increase in the demand for electricity in the summer and in the price for cooling capacity for the end user [1. Both factors have an adverse impact on large office buildings, shopping malls, sports and culture centers in cities as main customers for

System description
The District Heating System (DHS) subject to analysis supplies heat to a town of 20,000 inhabitants in central Poland. The DHS consists of a District Heating Plant (DHP) with total power of around 7 MW, District Heating Network (DHN) with total length of 3.1 km and 39 District Heating Substations (DHSu). The layout of the DHS is shown in Fig. 1  The current total heat demand of consumers is 5.0 MW and by category: 4.0 MW for CH and 1.0 MW for DHW purposes, but the real total heat demand is 3.4 MW (2.6 MW for CH and 0.8 MW for DHW). During the last three years, the annual amount of heat sold to consumers is 31 000 GJ and heat transportation losses are 10-11.5%, but during the summer season losses reach 25%. The quantity of degree-days in that period is in the range 3684.4-3981.0. Pressure disposal in the DHP varies in the range 180-220 kPa (average is 188 kPa), while water supply and return temperatures are in the range 70-94.1°C (average is 71.6°C), and 40-64.1°C (average is 46.5°C) respectively. The real value of total flow coefficient Kv of the DHN varies from 19.6 to 127.4 m 3 /h (average is 47.4 m 3 /h) and the drop in water temperature in the DHN is 11.8-31.5 K (average is 23°C).
Analyses of operational data have indicated that currently there is insufficient adaptation of operation conditions of the DHP to real consumer demand for heat for CH purposes. The biggest value of this discrepancy, which is characterized through the Coefficient of Heat Load (CHL), is below 0.43 and appears in the transition period with the outside temperature of +3°C, which is most of the time of the heating season. Figure 2 shows in schematic form the DHS with the Thermal Energy Storage (TES), which produces and distributes heat for both heating and cooling purposes. In the analyzed case, heat is produced in in the DHP furnished with 5 CB for CH and DHW purposes during the heating season, and chilled water for Air Conditioning (AC) and DHW during the summer season.

Application of ARS for DHS
In Poland, most DHS supply only heat to their clients, but during the summer season heat demand falls significantly in most urban areas which, taking into consideration the significantly lower price growth rate of heat in comparison e.g. to the electricity price growth rate, defines the potential use of DHS during the summer to power ARS [6].
Also benefiting from increasing heating demand in summer would be CHP plants, as they could boost electricity production due to the increased heat demand.

Calculation of hot water flow rate for supplying ARS
To give an idea of increased heat demand, an example is given below of the calculation of hot water flow increase for the purposes of supplying ARS with various cooling capacities from 500 kW to 1000 kW. For the purposes of the calculation some assumptions regarding the absorption chillers Coefficient of Performance (COP) were made [7,8]. To give greater insight, example calculations have been made using the following formula: The CF is applied to gain the hot water mass flow.

Operational analyses of DHS due to introduction of cooling generation
A detailed analysis of this tri-generation system was performed. The analysis covers the effect of increase in supply temperature, both on efficiencies of heat generation in CB and chilled water production in ARS. Also, changes in water flowrate, return temperature and pressure disposal were analyzed for water supply temperatures in the range 70°C to 85°C. Efficiency of heat generation in condensing type gas boilers was approximated on the basis of real experimental data through polynomial in the following form: where: = ( − )/2 Useable power of DHP was calculated from the equation: Heating power balance for the system could be expressed in the following way: Required power to generate DHW could be calculated as: Losses of power from supply DHN were calculated from the equation: Losses of power from return DHN are the following: Required heating power of ARS was estimated by heat effects in cooling spaces:  As a result of increase in supply water temperature Ts for DHS, increase in heat losses from supply piping is evident. Simultaneously, the following can be observed: decrease in required pressure disposal for DHS and in flow rate of water supplying DHSu (Fig. 3). Simultaneously, the working conditions and effectiveness of chilled water production by absorption chillers are improved [9][10][11]. Fig. 4 presents changes in efficiency of heat generation in the CB and cool generation in absorption chillers vs. Coefficient of Heat Load (CHL).

Analysis of heat supply for chilled water production purposes
Seasonal, simulation calculations of heat demand for chilled water production by absorption chillers were carried out for habitable-office building. Fig. 5 shows changes in flowrate of water and heat demand for DHW and chilled water for the analyzed building on 8 selected days in May.

Operation of TES and its influence on DHS
Operational analyses of the DHS were performed for the purpose of selecting the appropriate size of the TES in order to increase the operational efficiency of the existing CB especially for DHW and ARS. Investment costs for the ARS and the TES system application in the DHS were also assessed [12].

Fig. 5.
Hourly changes in heat demand for chilled water production, cool demand and total flowrate of network water for the analyzed building and eight selected days in May.
Operational analysis of the DHS covered supply and demand side, i.e., heat generation by the CB and heat consumption by existing consumers for DHW purposes and new consumers for ARS purposes. Heat consumption analysis for the summer season was carried out in order to increase operational efficiency of the ARS installation and to investigate the application of TES systems in the DHS. Heat storage in the DHS and its effect on power and operation of the CB was investigated i.e., heat accumulation by (i) the DHN, and (ii) the non-pressure TES system was analyzed [13].
Finally, the calculation results were shown for the required capacity of the TES systems assuring continuous and efficient operation of the CB installation in the DHS, especially during the summer season. Also presented were some advantages of implementing the TES system in the DHS in the case where the CB and ARS are applied. Simulation of the DHS with the TES system using real, operational data for the system situated in central-southern Poland (real total heat demand is 3.4 MW i.e. 2.6 MW for CH and 0.8 MW for DHW) was performed to determine the optimum capacity of the TES tank from an operational point of view. The optimum capacity of the TES tank from an operational point of view means that the boilers can operate for a long time practically at constant capacity. The DHS heat demand fluctuation can be covered by the TES system. For the optimum operating conditions of the CB the volume of the TES system is approx. VTES = 55 m 3 . Changes in heating capacity of the CB and heat demand by the DHS during 2 weeks' operation in the summer season for boilers with total capacity of 970 kW are shown in Fig. 6.
Changes in the charging and discharging processes of the TES system for optimum operational heating capacity of the CB were analyzed. Fig. 6 shows also changes in the charging and discharging processes of the TES taking into consideration the capacity of the DHN, during 2 weeks' operation in the summer season for optimum boiler power.

Influence of cooling generation on conditions of DHS
Results of detailed analysis show that additional heat production for chilled water purposes has a major effect on technological and economical operation of the existing system, especially in the summer season. the required condition to increase water supply temperature Ts, causes increased total heat losses of the DHN, slightly decreased efficiency of heat production by the CB and increased fuel consumption. Influence of cooling generation on operational and economic conditions of DHS is shown in Fig. 7. Moreover, existing infrastructure and flow capacity of DHN will be much better utilized when additional heat production for chilled water starts, e.g., total heat losses of DHN will drop from the current 25% to 19%. Taking into consideration the present condition of heat generation for DHW purposes in the summer season, when the supply temperature of network water Ts is 70°C, and the new condition when Ts is raised to 85°C due to chilled water production, the following items shown in Table 1 will change.

Conclusions
In this paper, computer simulations were performed for different supply temperature Ts in DHN, assuming as input, real changes in the basic parameters of the DHS (heat demand for DHW and ARS, flow rates, pressure disposal). On the basis of the above-mentioned calculations and taking into account investment costs of the ARS and the TES system, an optimal size (capacity) of the TES system was proposed for the considered DHS. Finally, the capacity of the TES system was 55 m 3 ensuring smooth and efficient operation of the CB. Application of the ARS with the TES system in the DHS in question increases net profit by 19.4%, reducing the cooling price for consumers by 40%from 0.60 PLN/kWh to 0.35 PLN/kWh. The existing CB can operate with high constant efficiency in the wide range of changes in DHS heat demand. Expected advantages of applying the ARS and TES system in the DHS are as follows:  increase in overall, operational efficiency of heat generation, by operation of the CB close to optimal conditions of the units,  decrease in fuel consumption due to continuous operation of the CB with capacity within the limits of high efficiency,  decrease in the emission of pollutants to the atmospheric air,  smooth and continuous operation of the CB, which leads to higher availability of the unit and increased failure-free operation time,  decrease in the percentage of heat losses from DHS, especially during the summer season (from 19% to 25%),  decrease in the percentage of water losses in the DHS related to water expansion with temperatures changes (the TES can additionally play the role of an expansion tank).