Energy and Exergy Analysis of Sensible Thermal Energy Storage—Hot Water Tank for a Large CHP Plant in Poland †

paper an extended version of paper: Improvement of operation of steam cushion system for sensible Abstract: The paper contains a simpliﬁed energy and exergy analysis of pumps and pipelines system integrated with Thermal Energy Storage (TES). The analysis was performed for a combined heat and power plant (CHP) supplying heat to the District Heating System (DHS). The energy and exergy e ﬃ ciency for the Block Part of the Siekierki CHP Plant in Warsaw was estimated. CHP Plant Siekierki is the largest CHP plant in Poland and the second largest in Europe. The energy and exergy analysis was executed for the three di ﬀ erent values of ambient temperature. It is according to operation of the plant in di ﬀ erent seasons: winter season (the lowest ambient temperature T ex = − 20 ◦ C, i.e., design point conditions), the intermediate season (average ambient temperature T ex = 1 ◦ C), and summer (average ambient temperature T ex = 15 ◦ C). The presented results of the analysis make it possible to identify the places of the greatest exergy destruction in the pumps and pipelines system with TES, and thus give the opportunity to take necessary improvement actions. Detailed results of the energy-exergy analysis show that both the energy consumption and the rate of exergy destruction in relation to the operation of the pumps and pipelines system of the CHP plant with TES for the tank charging and discharging processes are low.


Introduction
Nowadays, heat production for heating or domestic hot water is carried out in various ways. The heat can be produced locally, e.g., by gas or electric heaters, biomass boilers [1][2][3], etc., applicable to individual buildings or apartments [4,5]. Heat can also be produced ecologically using renewable energy sources [6]. Examples of this are solar collectors [7,8] and geothermal sources [9][10][11][12][13][14]. Heat can be also produced on a large scale, e.g., for a whole city (so called District Heating System-DHS) [15][16][17][18][19]. For this purpose, district heating plants (DHP) or combined heat and power plants (CHP) producing electricity and heat in the so-called cogeneration are used.
Heat storage is an important aspect in the functioning of today's energy sector. This is related, inter alia, to the dynamic development of renewable energy, and hence to the uneven generation of electricity in relation to the demand. Heat is a by-product in the functioning of large power plants

System Description
Energy and exergy analysis was performed for the Blocks Part of CHP plant Siekierki in Warsaw. CHP Plant Siekierki is the largest CHP plant in Poland and the second largest in Europe. The heating capacity of this plant is 2078 MW th whereas electrical capacity is 622 MW el . CHP plant Siekierki was launched in 1961. It consists of: • Collector Part-the boilers supply a common steam collector from which the steam is directed to the turbines (number of boilers-4, number of turbines-5, electrical capacity-170 MW el ). • Blocks Part-each block consists of its own boiler and its own turbine (number of blocks-3, electrical capacity-110 MW el each, heating capacity-175 MW th each).
The Blocks Part of the Siekierki Combined Heat and Power Plant consists of base load units. During the heating season, all blocks work as basic units in continuous mode. Only one heating block is in operation in summer. This is due to the much lower heat demand. Heat is only needed to provide domestic hot water. When the ambient temperature drops below zero, the collector part is introduced to operation. When the temperature drops below −15 • C, the water boilers are started. They are considered as a peak load boilers. Some cogeneration units are also put into operation when heat demand is low or does not exist. This takes place when the electricity demand is high or electricity prices are profitable. The decisive factor here is the economic calculation. Figure 1 shows a simplified layout of CHP plant Siekierki. The CHP plant is connected to the DHS. The main piping lines indicated in the Figure 1 as O, C, L, and U are responsible for supplying hot water to the DHS (top of the figure) and collecting cold water coming back (return) from DHS (bottom of the figure). The pumps responsible for return water are indicated as RP, pumps supplying water to the DHS are indicated as SP, water boilers B, and network water heaters XB or XC.
Energies 2020, 13, x FOR PEER REVIEW 3 of 16 conditions), intermediate season (average temperature Tex = 1 °C), and summer season (average temperature Tex = 15 °C) using hourly operational data. Such detailed analysis of implementation of the TES to the CHP plant and DHS is quite unique and can be considered as novel. This approach offers real conclusions resulting from energy and exergy analysis for future investors to furnish CHP plants with a TES system.

System Description
Energy and exergy analysis was performed for the Blocks Part of CHP plant Siekierki in Warsaw. CHP Plant Siekierki is the largest CHP plant in Poland and the second largest in Europe. The  Only one heating block is in operation in summer. This is due to the much lower heat demand. Heat is only needed to provide domestic hot water. When the ambient temperature drops below zero, the collector part is introduced to operation. When the temperature drops below −15 °C, the water boilers are started. They are considered as a peak load boilers. Some cogeneration units are also put into operation when heat demand is low or does not exist. This takes place when the electricity demand is high or electricity prices are profitable. The decisive factor here is the economic calculation. Figure 1 shows a simplified layout of CHP plant Siekierki. The CHP plant is connected to the DHS. The main piping lines indicated in the Figure 1 as O, C, L, and U are responsible for supplying hot water to the DHS (top of the figure) and collecting cold water coming back (return) from DHS (bottom of the figure). The pumps responsible for return water are indicated as RP, pumps supplying water to the DHS are indicated as SP, water boilers B, and network water heaters XB or XC.   TES at the Siekierki CHP plant was put into operation in March 2009. Its design process started in 2007 and it was built in 2008. Its design is pressure-free tank. The TES was integrated with the DHS system by Discharging Pumps (DP). A steam cushion system has been used to prevent oxygen absorption by the water in the reservoir.
The TES tank is shown in Figure 2 and the basic technical parameters of the TES are presented in Table 1.
in Table 1.

Materials and Methods
Energy and exergy analysis was undertaken for three different operation variants for the Blocks Part of the CHP plant [31]: • operation without TES, • operation with TES-charging process, • operation with TES-discharging process.
For all three operation variants the calculations for three representative values of outside temperatures were provided, i.e., Tex = −20 °C-calculated temperature for the District Heating System in this region of Poland, Tex = 1 °C-temperature close to the average temperature for the heating season, Tex = 15 °C-average temperature during the summer season.
As input data for the calculations, operating parameters were collected from the existing plant (CHP plant Siekierki Warsaw). The data are hourly average taken from the DCS system for the period of 2 years before the TES installation and for 2 years after the installation. Data were measured and collected from characteristic CHP plant points which are indicated as numbers 1-7 presented in

Materials and Methods
Energy and exergy analysis was undertaken for three different operation variants for the Blocks Part of the CHP plant [31]: • operation without TES, • operation with TES-charging process, • operation with TES-discharging process.
For all three operation variants the calculations for three representative values of outside temperatures were provided, i.e., T ex = −20 • C-calculated temperature for the District Heating System in this region of Poland, T ex = 1 • C-temperature close to the average temperature for the heating season, T ex = 15 • C-average temperature during the summer season.
As input data for the calculations, operating parameters were collected from the existing plant (CHP plant Siekierki Warsaw). The data are hourly average taken from the DCS system for the period of 2 years before the TES installation and for 2 years after the installation. Data were measured Energies 2020, 13, 4842 5 of 16 and collected from characteristic CHP plant points which are indicated as numbers 1-7 presented in . The parameters that were measured are as follows: temperatures, flows, and pressures (return and supply water). The remaining quantities necessary for energy and exergy analysis were calculated.
Heating capacity for XA heat exchanger is expressed as: Heating capacity for XB heat exchanger is stated as: Power of SP pumps (η = 0.82; three devices) is described as:   of the water at seven points of the system for block numbers 7, 9, and 10 were calculated on the basis of operational data of the CHP plant collected during the charging process of the TES (1-before RP pumps, 2-after RP pumps and before XA heat exchanger, 3-after XA and before XB heat exchangers, 4-after XB heat exchanger and before SP pumps, 5-after SP pumps, 6-cold side of TES before control valve, 7-cold side of TES after control valve). The power of the pumps RP and SP and heating capacities of the heat exchangers XA and XB were calculated from the same Equations (1)-(4) as for operation of the plant without TES and also for various external air temperatures: Tex = −20 °C, Tex = 1 °C, and Tex = 15 °C  Figure 5 shows the pressures diagram for the Blocks Part of CHP plant Siekierki during operation of the plant with TES in the course of the discharging process of the TES. The thermodynamic parameters of the water at seven points of the system for block numbers 7, 9, and 10 were calculated on the basis of operational data of the CHP plant collected during the discharging process of the TES (1-before RP pumps, 2-after RP pumps and before XA heat exchanger, 3-after XA and before XB heat exchangers, 4-after XB heat exchanger and before SP pumps, 5-after SP pumps, 6-cold side of TES after DP pump, 7-cold side of TES before DP pump).

Exergy Analysis
Exergy is defined as the maximum amount of work that a thermodynamically open system can do in a given environment by going into equilibrium with the environment. The environment is treated as a reservoir of useless energy and matter at a constant temperature. Maximum energy is obtained in a reversible process [27]. Exergy analysis can be recognized as one of the most important methods for performance evaluations and design calculations of TES systems. It is considered as more powerful than energy analysis [32][33][34][35].
In this paper exergy analysis is simplified and neglects exergy destruction for the storing period of the TES. This is related to the fact that the analyzed TES in Warsaw CHP plant has the following construction and operation characteristics: • the tank insulation is 500 mm thick (glass wool), therefore heat losses to ambient air are very low, • the storing periods are short, not usually more than a few hours, • stratification and thermocline are observed as good and very stable.
The heat losses from the TES tank depend mainly on the thickness of insulation, insulation conductivity, and the heat transfer coefficient [36]. For sensible water TES with a short-term storage period, 500 mm thickness of insulation is commonly applied. Presently, as insulation material, usually glass wool is preferred due to its low density and thermal conductivity. In case of such insulated water tank, heat losses do not exceed 1-3% of the total heat stored in the tank as was observed during start-up and commissioning of the TES in the last few years in Poland [31].
Results of analyses presented in [33,37] indicate that for properly designed TES, the storing period is characterized by relatively high exergy efficiencies in excess of 80%.
In order to integrate the heat accumulator into the existing hydraulic system of the CHP plant, new pipelines were added. The length of new pipelines does not exceed 5% of the length of existing pipelines. Exergy losses in pipelines should be considered as an important element of the balance of losses for the entire system, however, due to a slight change in the existing hydraulic system, it was decided to omit these calculations in this study.
As was presented in [33,38,39], the three-zone temperature-distribution models for the TES tank appear to provide sufficient calculation accuracy for exergy contents of vertically stratified TES. Additionally, in this paper [33], a stepped (two-zones) temperature-distribution model was analyzed

CHP Plant Operation without TES
The pressures diagram for the Blocks Part of the CHP plant Siekierki during operation of the plant without TES is shown in Figure 3. The thermodynamic parameters of water at five points of the system for block numbers 7, 9, and 10 were calculated on the basis of collected operational data of CHP plant (1-before RP pumps, 2-after RP pumps and before XA heat exchanger, 3-after XA and before XB heat exchangers, 4-after XB heat exchanger and before SP pumps, 5-after SP pumps).
The power of the pumps RP and SP and heating capacities of the heat exchangers XA and XB were calculated from following equations for various external air temperatures: T ex = −20 • C, T ex = +1 • C, and T ex = +15 • C: Power for RP pumps (η = 0.8; three devices) is calculated as: Heating capacity for XA heat exchanger is expressed as: .
Heating capacity for XB heat exchanger is stated as: Power of SP pumps (η = 0.82; three devices) is described as: of operational data of the CHP plant collected during the charging process of the TES (1-before RP pumps, 2-after RP pumps and before XA heat exchanger, 3-after XA and before XB heat exchangers, 4-after XB heat exchanger and before SP pumps, 5-after SP pumps, 6-cold side of TES before control valve, 7-cold side of TES after control valve).
The power of the pumps RP and SP and heating capacities of the heat exchangers XA and XB were calculated from the same Equations (1)-(4) as for operation of the plant without TES and also for various external air temperatures: T ex = −20 • C, T ex = 1 • C, and T ex = 15 • C 3.1.3. CHP Plant Operation with TES-Discharging Process of TES Figure 5 shows the pressures diagram for the Blocks Part of CHP plant Siekierki during operation of the plant with TES in the course of the discharging process of the TES. The thermodynamic parameters of the water at seven points of the system for block numbers 7, 9, and 10 were calculated on the basis of operational data of the CHP plant collected during the discharging process of the TES (1-before RP pumps, 2-after RP pumps and before XA heat exchanger, 3-after XA and before XB heat exchangers, 4-after XB heat exchanger and before SP pumps, 5-after SP pumps, 6-cold side of TES after DP pump, 7-cold side of TES before DP pump).
The power of the pumps RP and SP and heating capacities of the heat exchangers XA and XB were calculated from the same Equations (1)-(4) as for operation of the plant without TES. The power of the DP pumps was calculated from Equation (5). As in previous cases all calculations were performed for various external air temperatures: Power of TES pumps (DP) is calculated as:

Exergy Analysis
Exergy is defined as the maximum amount of work that a thermodynamically open system can do in a given environment by going into equilibrium with the environment. The environment is treated as a reservoir of useless energy and matter at a constant temperature. Maximum energy is obtained in a reversible process [27]. Exergy analysis can be recognized as one of the most important methods for performance evaluations and design calculations of TES systems. It is considered as more powerful than energy analysis [32][33][34][35].
In this paper exergy analysis is simplified and neglects exergy destruction for the storing period of the TES. This is related to the fact that the analyzed TES in Warsaw CHP plant has the following construction and operation characteristics: • the tank insulation is 500 mm thick (glass wool), therefore heat losses to ambient air are very low, • the storing periods are short, not usually more than a few hours, • stratification and thermocline are observed as good and very stable.
The heat losses from the TES tank depend mainly on the thickness of insulation, insulation conductivity, and the heat transfer coefficient [36]. For sensible water TES with a short-term storage period, 500 mm thickness of insulation is commonly applied. Presently, as insulation material, usually glass wool is preferred due to its low density and thermal conductivity. In case of such insulated water tank, heat losses do not exceed 1-3% of the total heat stored in the tank as was observed during start-up and commissioning of the TES in the last few years in Poland [31].
Results of analyses presented in [33,37] indicate that for properly designed TES, the storing period is characterized by relatively high exergy efficiencies in excess of 80%.
In order to integrate the heat accumulator into the existing hydraulic system of the CHP plant, new pipelines were added. The length of new pipelines does not exceed 5% of the length of existing pipelines. Exergy losses in pipelines should be considered as an important element of the balance of losses for the entire system, however, due to a slight change in the existing hydraulic system, it was decided to omit these calculations in this study.
As was presented in [33,38,39], the three-zone temperature-distribution models for the TES tank appear to provide sufficient calculation accuracy for exergy contents of vertically stratified TES. Additionally, in this paper [33], a stepped (two-zones) temperature-distribution model was analyzed and the results were compared with a basic three-zone temperature-distribution model. The equivalent temperature of a mixed TES that has the same exergy as the stratified TES was calculated for both of the above-mentioned models. The difference between temperatures computed for those models was less than 1%, therefore for further calculation of exergy destruction for the TES tank, a stepped two-zones model was applied, for reasons of greater simplicity.

CHP Plant Operation without TES
For the scheme presented in Figure 3 thermodynamic parameters at specified points of the system for block numbers 7, 9, and 10 were calculated on the basis of collected operational data of the CHP plant without TES [32]. Water and steam parameters were calculated using IF-97 formulas [40].
In pumps, exergy destruction is described by following equation (Gouy-Stodola Theorem) [41]. Due to the fact that the heat transfer in this process is assumed to be zero, the equation takes the form as below: The Gouy-Stodola Theorem states that the rate of exergy destruction is proportional to the rate of entropy generation. This destruction is caused by irreversibility and is equal to the ambient temperature multiplied by the sum of the increases in entropy of all components participating in the thermodynamic transformation. The exergy losses calculated according to this equation are additive. The exergy loss described by the Gouy-Stodola Theorem is completely irreversible and cannot even be partially recovered.
Exergy destruction in RP pumps (three devices): Energies 2020, 13, x FOR PEER REVIEW 8 of 16 and the results were compared with a basic three-zone temperature-distribution model. The equivalent temperature of a mixed TES that has the same exergy as the stratified TES was calculated for both of the above-mentioned models. The difference between temperatures computed for those models was less than 1%, therefore for further calculation of exergy destruction for the TES tank, a stepped two-zones model was applied, for reasons of greater simplicity.

CHP Plant Operation without TES
For the scheme presented in Figure 3 thermodynamic parameters at specified points of the system for block numbers 7, 9, and 10 were calculated on the basis of collected operational data of the CHP plant without TES [32]. Water and steam parameters were calculated using IF-97 formulas [40].
In pumps, exergy destruction is described by following equation (Gouy-Stodola Theorem) [41]. Due to the fact that the heat transfer in this process is assumed to be zero, the equation takes the form as below: (6) The Gouy-Stodola Theorem states that the rate of exergy destruction is proportional to the rate of entropy generation. This destruction is caused by irreversibility and is equal to the ambient temperature multiplied by the sum of the increases in entropy of all components participating in the thermodynamic transformation. The exergy losses calculated according to this equation are additive. The exergy loss described by the Gouy-Stodola Theorem is completely irreversible and cannot even be partially recovered.
Exergy destruction in RP pumps (three devices): Exergy destruction in XA heat exchanger (necessary parameters of medium, i.e., steam, condensate, and water, were calculated for the points shown in the sketch) was calculated as per the procedure shown below.
Power of XA heat exchanger: -given from operational data (8) Steam/condensate mass flow: Water mass flow: -was given from operational data (10) Entropy change: Exergy destruction: Exergy destruction in XA heat exchanger (necessary parameters of medium, i.e., steam, condensate, and water, were calculated for the points shown in the sketch) was calculated as per the procedure shown below.
Energies 2020, 13, x FOR PEER REVIEW 8 of 16 and the results were compared with a basic three-zone temperature-distribution model. The equivalent temperature of a mixed TES that has the same exergy as the stratified TES was calculated for both of the above-mentioned models. The difference between temperatures computed for those models was less than 1%, therefore for further calculation of exergy destruction for the TES tank, a stepped two-zones model was applied, for reasons of greater simplicity.

CHP Plant Operation without TES
For the scheme presented in Figure 3 thermodynamic parameters at specified points of the system for block numbers 7, 9, and 10 were calculated on the basis of collected operational data of the CHP plant without TES [32]. Water and steam parameters were calculated using IF-97 formulas [40].
In pumps, exergy destruction is described by following equation (Gouy-Stodola Theorem) [41]. Due to the fact that the heat transfer in this process is assumed to be zero, the equation takes the form as below: (6) The Gouy-Stodola Theorem states that the rate of exergy destruction is proportional to the rate of entropy generation. This destruction is caused by irreversibility and is equal to the ambient temperature multiplied by the sum of the increases in entropy of all components participating in the thermodynamic transformation. The exergy losses calculated according to this equation are additive. The exergy loss described by the Gouy-Stodola Theorem is completely irreversible and cannot even be partially recovered.
Exergy destruction in RP pumps (three devices): Exergy destruction in XA heat exchanger (necessary parameters of medium, i.e., steam, condensate, and water, were calculated for the points shown in the sketch) was calculated as per the procedure shown below.
Power of XA heat exchanger: -given from operational data (8) Steam/condensate mass flow: Water mass flow: -was given from operational data (10) Entropy change: Power of XA heat exchanger: . Q XA -given from operational data Exergy destruction: .
Exergy destruction in XB heat exchanger is calculated in a similar way as for heat exchanger XA: power of XB heat exchanger: . Q XB -was given from operational data (13) Steam/condensate mass flow: .
Water mass flow: . m w -was given from operational data (15) Entropy change: Exergy destruction: .
Exergy destruction in SP pumps (three devices): Energies 2020, 13, x FOR PEER REVIEW 9 of 16 (12) Exergy destruction in XB heat exchanger is calculated in a similar way as for heat exchanger XA: power of XB heat exchanger: -was given from operational data (13) Steam/condensate mass flow: Water mass flow: -was given from operational data (15) Entropy change: Exergy destruction: (17) Exergy destruction in SP pumps (three devices):

CHP Plant Operation with TES-Charging Process of TES
Throttling losses during the charging process: Exergy destruction for TES during the charging process:

CHP Plant Operation with TES-Discharging Process of TES
Exergy destruction in TES Discharging Pumps (DP pumps):

CHP Plant Operation with TES-Charging Process of TES
Throttling losses during the charging process: Exergy destruction for TES during the charging process: Energies 2020, 13, x FOR PEER REVIEW 9 of 16 (12) Exergy destruction in XB heat exchanger is calculated in a similar way as for heat exchanger XA: power of XB heat exchanger: -was given from operational data (13) Steam/condensate mass flow: Water mass flow: -was given from operational data (15) Entropy change: Exergy destruction: (17) Exergy destruction in SP pumps (three devices):

CHP Plant Operation with TES-Charging Process of TES
Throttling losses during the charging process: Exergy destruction for TES during the charging process:

CHP Plant Operation with TES-Discharging Process of TES
Exergy destruction in TES Discharging Pumps (DP pumps): .

CHP Plant Operation with TES-Discharging Process of TES
Exergy destruction in TES Discharging Pumps (DP pumps):

CHP Plant Operation with TES-Discharging Process of TES
Exergy destruction in TES Discharging Pumps (DP pumps): Exergy destruction for TES during the discharging process could also be calculated from Equation (20).
Exergy destruction for TES during the discharging process could also be calculated from Equation (20).

Results and Discussion
Results of the energy analysis for the Blocks Part of the CHP plant are presented in Table 2. This analysis was made for three different operation variants of the CHP plant, i.e., operation without TES, operation with TES-charging process of TES and operation with TES-discharging process of TES. Calculations were performed for three different outside temperatures: T ex = −20 • C, T ex = 1 • C, and T ex = 15 • C for each considered operation variant.  Table 2 indicate that in all cases, i.e., for each operation variant of the CHP plant and each different outside temperature, TES pumps power is relatively low in comparison to RP and SP pumps power and especially to XA and XB heat exchangers heating capacity. Figure 6 shows the percentage of pumps power for discharging of TES operation variant and outside temperatures T ex = −20 • C alternative. In that case TES pumps power represents only 2% of pumps total power in the Blocks Part of the CHP plant.

SP pumps
3.4 3.4 1.6 2.6 2.0 -2.9 2.9 -DP pumps 0.7 0.8 0.5 0.7 0.8 -0.7 0.8 - Figure 6 shows the percentage of pumps power for discharging of TES operation variant and outside temperatures Tex = −20 °C alternative. In that case TES pumps power represents only 2% of pumps total power in the Blocks Part of the CHP plant.  Tables 3 and 4. Table 3 contains exergy destructions of RP, SP pumps and XA, XB heat exchangers  Tables 3 and 4. Table 3 contains exergy destructions of RP, SP pumps and XA, XB heat exchangers for block numbers 7, 9, and 10 operating without TES and during the charging and discharging process of TES for different outside temperatures.

Conclusions
The paper contains a simplified energy and exergy analysis of the hydraulic system integrated with Thermal Energy Storage (TES). The analysis was performed for combined heat and power plant (CHP) supplying heat to the District Heating System (DHS). The energy and exergy analysis was performed for the winter season (the lowest ambient temperature Tex = −20 °C, i.e., design point conditions), the intermediate season (average ambient temperature Tex = 1 °C), and summer (average ambient temperature Tex = 15 °C). The presented results of the analysis make it possible to identify the places of the greatest exergy destruction in the pumps and pipelines system with TES, and thus give the opportunity to take necessary improvement actions in further steps. The analysis carried out as part of the article shows that in order to significantly reduce the consumption of electricity and heat in the considered hydraulic system, modernization work should be undertaken in the area of heat exchangers and pumps. In the case of pumps, the possibility of increasing the efficiency of the pumps (e.g., by replacing them with modern ones) and introducing their effective regulation (variable-speed regulation, i.e., introduction of frequency converters, etc.) should be considered. In the case of heat exchangers, replacing them, for example, with heat pumps, however, such an operation will require separate analysis and significant investment costs.Detailed results of the energy-exergy analysis show that both the energy consumption and the rate of exergy destruction in relation to the operation of the pumps and pipelines system of the CHP plant with TES for the tank charging and discharging cycles are relatively low, i.e., the power of the additional equipment (discharging pumps DP) is at the level of 0.5-0.8 MW which corresponds to about 0.5-0.8% of the

Conclusions
The paper contains a simplified energy and exergy analysis of the hydraulic system integrated with Thermal Energy Storage (TES). The analysis was performed for combined heat and power plant (CHP) supplying heat to the District Heating System (DHS). The energy and exergy analysis was performed for the winter season (the lowest ambient temperature Tex = −20 °C, i.e., design point conditions), the intermediate season (average ambient temperature Tex = 1 °C), and summer (average ambient temperature Tex = 15 °C). The presented results of the analysis make it possible to identify the places of the greatest exergy destruction in the pumps and pipelines system with TES, and thus give the opportunity to take necessary improvement actions in further steps. The analysis carried out as part of the article shows that in order to significantly reduce the consumption of electricity and heat in the considered hydraulic system, modernization work should be undertaken in the area of heat exchangers and pumps. In the case of pumps, the possibility of increasing the efficiency of the pumps (e.g., by replacing them with modern ones) and introducing their effective regulation (variable-speed regulation, i.e., introduction of frequency converters, etc.) should be considered. In the case of heat exchangers, replacing them, for example, with heat pumps, however, such an operation will require separate analysis and significant investment costs.Detailed results of the energy-exergy analysis show that both the energy consumption and the rate of exergy destruction in relation to the operation of the pumps and pipelines system of the CHP plant with TES for the tank charging and discharging cycles are relatively low, i.e., the power of the additional equipment (discharging pumps DP) is at the level of 0.5-0.8 MW which corresponds to about 0.5-0.8% of the

Conclusions
The paper contains a simplified energy and exergy analysis of the hydraulic system integrated with Thermal Energy Storage (TES). The analysis was performed for combined heat and power plant (CHP) supplying heat to the District Heating System (DHS). The energy and exergy analysis was performed for the winter season (the lowest ambient temperature T ex = −20 • C, i.e., design point conditions), the intermediate season (average ambient temperature T ex = 1 • C), and summer (average ambient temperature T ex = 15 • C). The presented results of the analysis make it possible to identify the places of the greatest exergy destruction in the pumps and pipelines system with TES, and thus give the opportunity to take necessary improvement actions in further steps. The analysis carried out as part of the article shows that in order to significantly reduce the consumption of electricity and heat in the considered hydraulic system, modernization work should be undertaken in the area of heat exchangers and pumps. In the case of pumps, the possibility of increasing the efficiency of the pumps (e.g., by replacing them with modern ones) and introducing their effective regulation (variable-speed regulation, i.e., introduction of frequency converters, etc.) should be considered. In the case of heat exchangers, replacing them, for example, with heat pumps, however, such an operation will require Energies 2020, 13, 4842 13 of 16 separate analysis and significant investment costs.Detailed results of the energy-exergy analysis show that both the energy consumption and the rate of exergy destruction in relation to the operation of the pumps and pipelines system of the CHP plant with TES for the tank charging and discharging cycles are relatively low, i.e., the power of the additional equipment (discharging pumps DP) is at the level of 0.5-0.8 MW which corresponds to about 0.5-0.8% of the total power of the block (each block has 110 MW e ). The exergy destruction is in the range of 0.12-0.45 MW.
It should be emphasized that the introduction of a heat accumulation system like TES does not significantly increase energy losses and exergy destruction in the analyzed cases, so this type of investment will not significantly increase the consumption of electricity (pumps) and heat (heat exchangers and TES). Obviously, the amounts of energy and exergy losses differ significantly for different ambient temperatures, i.e., CHP plant operation in winter, intermediate, and summer periods, but the percentage of these losses is practically constant.
Detailed conclusions resulting from energy and exergy analysis could be expressed as follows: • Power of the TES pumps is relatively low in relation to the power of the block and is approx. 500-800 kW, which equates to approx. 0.5-0.8% of the block electric power.

•
For the lowest considered outside temperature T ex = −20 • C TES pumps power represents only 2% of pumps total power in the Blocks Part of the CHP plant.

•
Differences in powers and losses of the RP and SP pumps are negligible during operation of the CHP plant without and with the TES.

•
The biggest exergy destruction appears in the heat exchangers, but the differences between them for the analyzed cases, i.e., during operation of the Blocks Part of CHP plant without the TES and the charging and discharging processes of the TES, are negligible.

•
Throttling losses during the charging process of the TES and exergy destruction in Discharging Pumps (TES Pumps) are also low and fluctuate in the range 120-450 kW depending on the considered case.
A simplified energy and exergy analysis was also carried out for CHP plants with technologically similar TES systems, i.e., CHP plant in Białystok and CHP plant in Bielsko Biała. CHP plant in Bialystok is equipped with a TES with a capacity of 12,800 m 3 , while CHP plant in Bielsko Biała with a TES with a capacity of 21,450 m 3 [29]. The results of the analysis indicate that the percentages of energy losses and exergy destruction for heat exchangers, pumps and TES tanks are analogous to those presented in this paper.