Open access peer-reviewed chapter

Heat Pump to Increase the Efficiency of a Geothermal Heating System in the City of Călimănești - Case Study

Written By

Laurentiu Constantin Lipan and Sorin Dimitriu

Submitted: July 19th, 2022 Reviewed: August 19th, 2022 Published: November 25th, 2022

DOI: 10.5772/intechopen.107252

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Abstract

Romania is one of the European countries that has a rich geothermal potential, the main uses of this resource being the spaces heating and thermal baths. Recently, concerns about increasing energy efficiency and limiting greenhouse gas emissions, have determined an increased attention to heating systems using geothermal water. These will play an important role in the future in developing sustainable energies and reducing the use of fossil fuels. The authors present a case study on a heating system in the city of Călimănești, initially using liquid fuel, which was modernized by using geothermal hot water for the preparation of the thermal agent. As geothermal water, with an initial temperature of 95°C, cannot be cooled below 50°C, the authors considered the possibility of using a heat pump to fully exploit the thermal potential of geothermal water, increase the capacity of the system and increase its energy efficiency. The implementation of the heat pump in the district heating system and the results expected to be obtained are discussed. It was considered that the heat pump works in parallel with the heating system, obtaining an increase in its capacity by approx. 60% and an increase in energy efficiency by approx. 30%.

Keywords

  • district heating
  • geothermal heat pump
  • geothermal heating system
  • energy efficiency
  • heat recovery
  • use of geothermal water

1. Introduction

Geothermal energy can be defined as the energy inside the earth, which generates geological phenomena on a planetary scale. The term used comes from the Greek words “geo” (earth) and “thermos” (heat) and is used nowadays to define that part of the earth’s energy that can be recovered and used by man to generate heat and power. Geothermal energy is characterized as a renewable and sustainable energy. The character of renewable is given by its continuous production inside the earth. During the operation of natural geothermal systems, the regeneration of geothermal energy takes place by heating the geothermal water to the same time scale at which it is extracted for use. In the usual case of dry hot rocks and hot water aquifers in sedimentary basins, energy recharging is done through a slow process of thermal conduction. The sustainability of a resource depends on its initial quantity, regeneration rate and consumption rate. Obviously, consumption can be sustained if the resource is regenerated faster than it is depleted. In this context, sustainable development involves the use of the resource so that through continuous regeneration it will allow its use by future generations. The degree of sustainability of geothermal energy is still high because, compared to the volume of resources and fast regeneration rate, humanity out of convenience, uses only a small part.

Natural geothermal springs have been used for heating and thermal baths since ancient times. Archeological discoveries suggest that the earliest uses of geothermal energy took place over 10,000 years ago in North America, where autochthone populations have used the hot springs in this area for both practical and spiritual purposes. The finding that hot mineral baths ameliorate or even cure some diseases, led to the consideration of these springs as sacred, endowed by gods with magical healing powers [1].

Evidence of this has also been found in the peoples of ancient Greece and Roman Empire. At the same time, evidence was also discovered that attested to more commonplace uses: heating of living spaces, hot baths and activities related to food preparation. Evidence that geothermal energy was used for heating dates to the first century (AD), being found in the Roman city of Pompeii. However, concerns about such uses of geothermal energy were initially limited to locations where hot geothermal water was naturally accessible in the form of springs [2].

Nowadays, the direct use of geothermal energy is reported and documented in at least 88 countries. The estimated capacity to be currently installed in these countries is approx. 108,000 MW, and the annual energy consumption of approx. 1,021,000 TJ (284,000 GWh/year), being oriented towards: geothermal heat pumps (58.8%), swimming pools and thermal baths (18%), direct space heating (16%), greenhouse heating (3.5%), uses in industrial processes (1.6%), aquaculture and fish farming (1.3%), drying of cereals (0.35%), other uses (0.45%). The highest consumptions were reported in order in China, USA, Sweden, Turkey, and Japan [3].

According to data reported at the World Geothermal Congress in 2020 (WGC 2020), only in recent years, 2015-2020, there has been an increase in the amounts of energy used from geothermal sources by approx. 27%. Concerns about the production of electricity in ORC installations that take heat from geothermal waters have also intensified, with five countries installing production capacity for the first time: Belgium (0.8 MW), Chile (48 MW), Croatia (16.5 MW), Honduras (35 MW) and Hungary (3 MW). Approximately 2650 wells were drilled in 42 countries and approx. $ 22.3 billion has been invested in geothermal projects [4].

Geothermal energy resources are located over a wide range of depths and can be in the form of hot water, steam, or hot rocks. Hot water aquifers with temperatures between 60°C and 100°C are the most suitable applications for space heating and agricultural systems. For these aquifers to be commercially interesting, they must be located at depths up to 2000 - 3000 m and have a temperature of at least 60°C. Areas with hot water under pressure with higher temperatures and areas with hot rocks are suitable as a source for electricity generation [5].

Traditionally, the heating of homes, office buildings and commercial spaces has been done with the help of local heat sources: stoves, fireplaces and hot water boilers using different types of fuels. These systems not only have low energy efficiency but are also powerful generators of carbon dioxide as well as various polluting suspensions. For urban areas with high population density, all studies conducted at national and international level have led to the conclusion that from the point of view of energy efficiency and environmental protection, district heating (DH) systems are advantageous [6].

The multiple and obvious advantages of district heating are: high energy efficiency; the possibility of using several types of fuels; use of residual energy resulting from industrial processes (hot water, steam); use of renewable resources (solar energy, geothermal water, biomass, biofuels, household waste and other combustible waste); simple operation by the consumer, which is not involved in fuel supply activities, maintenance and operation supervision; consumer safety, compared to individual sources; reduced pollution, by placing thermal energy sources outside the living area and achieving a low level of pollutant emissions and greenhouse gases; the possibility of applying local investment policies in the field of energy efficiency and improving the quality of the environment.

Despite all these major advantages, compared to the alternative of individual heating, the consumer connected to a DH system also faces a certain degree of limitation of thermal comfort, determined by how the system can respond to variable loads or to operate economically under load limitation. Nevertheless, the DH system solution provides the necessary heating and hot water at prices less than or equal to those offered by individual alternative solutions. District heating is a suitable solution for all sizes of networks, from a few buildings to the neighborhood or city level. This has strengthened of DH system position in many European countries in recent years [7, 8].

In the current conditions in which the amplification of global warming imposes firm and hard measures for limiting the greenhouse gas emissions by restricting the use of traditional fuels, the integration of geothermal resources in DH systems is a major requirement. Geothermal heating systems not only reduce or eliminate carbon dioxide emissions but, by eliminating the consumption of conventional fuels, provide consumers with heat at much lower prices, especially in the current context of the gas and oil crisis. Geothermal heat resources have been used in district heating systems for many years in Iceland and France. Other such installations appear in Germany, Hungary, Italy, Romania, Belgium, and the United Kingdom. During 2014, 30 PJ of heat were supplied from geothermal sources worldwide, of which 7.3 PJ in the European Union. However, these reserves appear to be somewhat underestimated, according to reports from the European GeoDH project. Given that about a quarter of the European Union’s population lives in urban areas where geothermal energy could be extracted and used, future geothermal heating systems appear as an efficient solution to the current problems of thermal energy supply [9].

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2. Geothermal heating system in the city of Călimănești, Romania

Romania has an important potential of geothermal energy sources, on its territory being identified several areas in which the geothermal potential is estimated to allow economic applications. They are located on an extensive area in western plain of Romania, in the middle of country, on the Olt River Valley north of Râmnicu Vâlcea, in the northern part of Bucharest city and south of Brăila city. Figure 1 shows the locations of these geothermal reservoirs in Romania. Geological surveys conducted before 1990 showed that the known geothermal potential in Romania is about 10PJ/year, of which only about two thirds is exploited. The approximately 80 functional wells can produce an annual amount of thermal energy of about 7PJ [10].

Figure 1.

The distribution of geothermal reservoirs on the Romanian territory [10].

In Romania, the temperature of “low enthalpy” geothermal sources, with exploitation by drilling-extraction is between 25°C and 60°C, and 60–125°C for the “mesothermal”. The economic drilling and extraction limit for geothermal waters was agreed for the depth of 3300 m and was reached in some areas of Romania, such as the Bucharest North - Otopeni geothermal basin and certain perimeters in Snagov and Balotești localities. Romania was ranked as the third country in Europe, after Greece and Italy, for its very high geothermal potential [11].

More than 80% of the wells are exploited artesian, 18 wells require anti-scaling chemical treatment, and 6 are used for reinjection. The main direct uses of the geothermal energy are space and district heating; bathing; greenhouse heating; industrial process heat; fish farming and animal husbandry [11].

This chapter presents a case study. The authors present a solution to increase the energy efficiency of the district heating system of the city of Călimănești, which works having as main source of heat the geothermal water extracted from the aquifer of Călimănesti – Căciulata - Cozia perimeter, located on the Olt River Valley. In this area, the geothermal water is provided by three drillings having more than 3000 m in depths, located on the right side of the Olt River, at about 1-2 km one from each other as presented in the Figure 2. The three existing drillings stand out deposits of medium enthalpy geothermal water (the temperature at the exit of the well is 92…95°C). The available flow volume of the three wells is 50.4 l/s, equivalent to a thermal potential of 13.2 MW, when the geothermal water is cooled to 30°C [12].

Figure 2.

Olt Valley working geothermal perimeter [12].

The drilling located in vicinity of Căciulata and Cozia localities (borehole 1008 and 1006) are used only for local heating needs. The geothermal water feeds a group of hotels and SPA treatment units, for heating, domestic hot water supply and thermal pools. The high thermal potential of the geothermal water leads to its direct exploitation. In the cold season, the geothermal water (having a temperature of 92…95°C) is cooled in a plate heat exchanger, producing the thermal agent (TA) for the district heating system. A second heat exchanger produces domestic hot water (DHW). The geothermal water, cooled in the two heat exchangers, feeds the thermal pool, after that being discharged in the Olt River at a temperature of about 30°C. In the warm season, the mass flow extracted is reduced, only the heat exchanger for domestic hot water and thermal pool being in use.

The third drilling is situated at 1,2 km from Călimăneşti, providing a volume flow of 18 l/s at the same temperature values 92…95°C. This locality, beside the tourists which are staying in hotels, has about 8600 permanent habitants; 15% of the habitants are living in apartments connected to a centralized system for thermal energy supply. In the cold season of 2019-2020, 461 apartments and residential houses, 9 public institutions and 47 economic operators were branched to DH system, which must ensure a thermal need of about 3500 kW for heating and about 500 kW for DHW supply (for the conventional climatic parameters) [13]. The DH system was initially designed with three thermal units, equipped with hot water boilers using light liquid fuel. The geothermal water from the nearby well was initially used only for the thermal energy supply of the SPA treatment units and thermal pools. The project of the DH system feeding with geothermal energy was started in 2002 with internal financing and was later supported by European funds. Initially, the project included all the three wells (1006, 1008 and 1009) to provide the centralized heating of Călimănesti town. Later, it was utilized only the available water from the well 1009, situated in vicinity of town. The available volume flow is of 18 l/s, from which about 8 l/s is utilized by a SPA center and a hotel; the rest of volume flow (about 10 l/s) being used in the DH system of Călimănesti. To use the geothermal water into the DH system, a geothermal heating station was built just near the geothermal well. The geothermal water produces, by using plate heat exchangers, the primary thermal fluid for the DH system, having a temperature of about 85°C. This primary thermal fluid serves to partially cover the heating demand and to completely cover the DHW preparation.

Figure 3 shows the drilling 1009 and the geothermal station that prepares the heating agent for the DH system. The geothermal water from the Olt Valley aquifer has a high combustible gas content, containing over 90% methane. As a result, before being introduced into the heat exchangers of the geothermal station, it is degassed, the collected gases being discharged into the atmosphere. Figure 4 shows the actual operating diagram of the geothermal station.

Figure 3.

The borehole 1009 and geothermal station (source: photo of the authors).

Figure 4.

The actual operating diagram of the geothermal station [12]. GB – Geothermal borehole; DT – Degassing tank; PU – Pumping units; DTP – Distribution thermal points; PHEH – Plate heat exchanger for heating; PHEW – Plate heat exchanger for DHW.

The primary thermal agent from the DH system of Călimănești town is returned to the geothermal station with a temperature around 40°C. For this reason, the geothermal water extracted from the drilling well, with a temperature of 95°C, cannot be cooled, in the heat exchangers of geothermal station, to a temperature below of about 50°C. Because the exploitation of geothermal aquifer is artesian, this water is discharged directly into the Olt River after a cooling until 30°C, from reason of aquatic environment protection. The cooling and discharge into the Olt River of the waste geothermal water, represents a heat loss of approx. 1/3 of its full thermal potential. The authors examined the possibility of recovering this heat loss by implementing in the thermal agent preparation circuit a heat pump that uses as heat source the waste geothermal water discharged from the geothermal station, cooling it from 50–30°C. In this way, the entire thermal potential of geothermal water is used, while also increasing the capacity of the DH system by growing the flow of thermal agent produced. Waste geothermal water can be discharged directly into the Olt River, without any negative impact on the aquatic environment. The functional diagram of the geothermal station, coupled with a mechanical vapor compression heat pump, is presented in Figure 5.

Figure 5.

The functional diagram of the geothermal station with heat pump (source: the drawing of authors). CP- compressor; EXV – Expansion valve; CD – Condenser; EV – Evaporator.

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3. The geothermal heat pump

The idea of introducing a heat pump in the functional diagram of the geothermal station was to connect it in parallel with the heat exchangers that prepare the heat sent to the network of the DH system, supplementing its flow. The flow of geothermal water available from the wellbore is 10 l/s, which allows the coverage of a thermal load of 1820 kW, the geothermal water being cooled only to 50°C. Using the heat pump for further cooling of the geothermal water until a temperature of 30°C, an additional thermal power of 1069 kW can be obtained, estimating a value of the heat pump COP of 4,5. This result shows that the capacity of the district heating system is increased by about 60%. For geothermal wastewater to be used as a heat source, if it is cooled to 30°C, the refrigerant vaporization temperature must be around 25°C. As the temperature of the agent supplied to the heating system by the geothermal station is 85°C, the refrigerant used by the heat pump must have a condensation temperature of at least 90°C. Consequently, it is necessary to use high temperature working agents with a critical temperature above 100°C. In addition to this requirement, the refrigerant used must have an environmental impact in accordance with the provisions of the EU Regulation on fluorinated greenhouse gases and have thermodynamic properties to achieve the highest possible coefficient of performance.

Most of the agents shown in Table 1 are “wet agents”, for which the end of dry saturated vapors compression, from the evaporator, falls within the saturation domain. To avoid this phenomenon, a superheat of the vapors must be introduced at the compressor suction, of at least 10 degrees. The cooling of the heat pump condenser being carried out with the returned agent from the DH system, having a temperature of 45°C, a subcooling of refrigerant until a temperature around 60°C, is possible. Figure 6 shows how to perform the processes of the condensate subcooling and superheating of cold vapors at the compressor suction. The cooling is carried out either in a separate heat exchanger or in the final part of the condenser and the superheating of the vapors, by introducing an internal regenerative heat exchanger in the operation scheme of the heat pump.

RefrigerantGrouppsat at 20°C (bar)tcrit (°C)ODPGWP/CO2Safety class
R134aHFC6.654101.001430A1
R245faHFC1.486154.001030B1
R236faHFC2.719124.909810A1
R152aHFC5.979113.20138A2
R600aHC3.507134.903A3
R600HC2.433154.204A3
RE170HC6.908127.203A3
R515b*HFO4.974108.90293A1
R1234ze(E)HFO4.985109.401A2L
R1233zd(E)HFO1.298165.501A1

Table 1.

The properties of a few agents used for high temperature heat pumps [14].

Azeotropic blend R1234ze/R227ea (91,1%/8,9%).


Note: HFC = hydrofluorocarbons; HC = Hydrocarbons; HFO = Hydrofluoro-olefins.

Figure 6.

The operating diagram of the heat pump installation (source: the drawing of authors). EV – Evaporator; IHE – Internal heat exchanger; CP – Compressor; CD – Condenser; SR – Subcooler; EXV – Expansion valve; DTP – Distribution thermal point.

Subcooling of the condensate has the effect of increasing the specific thermal load of the condenser, respectively of the energy efficiency (COP). With the agents presented in Table 1, the authors performed the analysis of the thermodynamic cycle of the heat pump, to choose the most suitable agent for the imposed operating conditions:

  • heating of the thermal agent in the condenser: from 45–85°C;

  • condensate subcooling temperature (tsc): 60°C;

  • cooling of the geothermal water in the evaporator: from 50–30°C;

  • vapor overheating at the compressor suction (∆tsh): 10 degrees;

  • pinch point heat exchangers (∆tgap): around 5 degrees;

  • the isentropic efficiency of the compressor (ηc): 0.8.

For all the considered agents, according to these conditions, the condensation temperature was chosen tc=90°C., and the vaporization temperature tv=25°C

The thermodynamic cycle, in the p-h diagram, is presented in Figure 7.

Figure 7.

The thermodynamic cycle of the heat pump installation, in p-h diagram (source: generated in EES software by authors).

Saturation pressures corresponding to these temperatures depend on the agent considered:

pv=psattv;pc=psattcE1

The superheating of the vapors sucked by the compressor is carried out with the help of the hot condensate having a temperature of 60°C, by means of the regenerative heat exchanger. Since in this heat exchanger, the flow of the hot fluid is equal to that of the cold fluid, the thermal balance of the appliance has the form:

h4h5=h1h7E2

specifying the enthalpy of condensate at the inlet to the expansion valve.

The state parameters in the characteristic points of the operating scheme in Figure 6, respectively of the thermodynamic cycle in the p-h diagram in Figure 7, were determined using the EES software. The Table 2 shows the calculation algorithm of the state parameters of the cycle.

State No.State descriptionKnown state parametersFunctions for determining state parameters
1Superheated vapors at the compressor suctionp1=pv
t1=tv+tsh
h1=enthalpypvt1
s1=entropypvt1
sSuperheated vapors at the end of isentropic compressionp2s=pc
s2s=s1
t2s=temperaturepcs2s
h2s=enthalpypct2s
2Superheated vapors at the end of real, irreversible compressionp2=pc
h2=h1+h2sh1/ηc
t2=temperaturepch2
s2=entropypch2
3Liquid at saturationp3=pc
t3=tc
x3=0
h3=enthalpypcx3
s3=entropypcx3
4Subcooled liquidp4=pc
t4=tsc
h4=enthalpypctsc
s4=entropypctsc
5Subcooled liquid at the inlet of the expansion valvep5=pc
h5=h4+h1h7
t5=temperaturepch5
s5=entropypch5
6Two-phase mixture after expansion valvep6=pv
t6=tv
h6=h5
s6=entropypvh6
x6=qualitypvh6
7Dry saturated vapors at the exit of the evaporatorp7=pv
t7=tv
x7=1
h7=enthalpypvx7
s7=entropypvx7

Table 2.

Algorithm for determining the state parameters in EES software.

According to the thermodynamic cycle in Figure 7, the specific energy parameters of the installation are:

specific thermal load of the condenser:

qc=h2h4kJ/kgE3

specific cooling power of the evaporator:

qv=h7h6kJ/kgE4

specific mechanical compression work:

lcp=h2h1kJ/kgE5

The heat flow taken up by the heat pump evaporator depends on the available geothermal water flow V̇gw [m3/s] and the temperature up to which the water can be cooled:

Q̇v=V̇gwρgwhgw1hgw2kWE6

where ρgw [kg/m3], the density of waste geothermal water, was considered at its average temperature tgw=0.5tgw1+tgw2. The flow of refrigerant is determined by the heat flow recovered from waste geothermal water:

ṁ=Q̇vqvkg/sE7

and establishes the heat flow given to the condenser and respectively the theoretical power required to drive the compressor:

Q̇c=ṁqckWE8
Pcp=ṁlcpkWE9

resulting in COP (energy efficiency)

COP=Q̇cPcpE10

The efficiency of the Carnot cycle carried out between the two heat sources of the thermodynamic cycle, the hot source, and the cold source, has the expression:

COPC=THSTHSTCSE11

wherein the temperatures of the two heat sources, THS and TCS, are the mean thermodynamic temperatures of the hot water produced at the condenser and respectively of the geothermal water from which the heat is extracted at the evaporator.

The mechanical work of the reversible Carnot cycle has the significance of the minimum mechanical work necessary to achieve heat transfer from cold to hot source. In the case of a real irreversible cycle, carried out between the same heat sources, the internal and external irreversibility, determine a destruction of the available energy, determining the increase of the mechanical work consumption. The most crucial parameter for the thermodynamic evaluation of a cycle, from this point of view, is the exergetic efficiency (carnotic efficiency):

ηex=lClcycle=1π¯ir,int+π¯ir,ext=COPCOPCE12

wherein π¯ir,int is the sum of the rates of the losses caused by the internal irreversibility of the cycle and π¯ir,ext, the sum of the rates of the losses caused by its external irreversibility.

To determine which of the working agents shown in Table 1 is more suitable for this heat pump installation, in terms of properties and performance, were determined the state parameters for each point of the cycle in Figure 7, according to the calculation algorithm presented in Table 2. The calculus was carried out in the same initial conditions for each refrigerant. The drive power of the compressor and the heat flow yielded to the condenser were also calculated, resulting in the performance coefficient of the installation. The reference Carnot cycle was considered between the average thermodynamic temperatures of the geothermal water in the evaporator and respectively of the thermal agent in the condenser.

The results obtained are presented in Table 3 and in Figure 8.

Refrigerantqv(kJ/kg)qc(kJ/kg)lcp(kJ/kg)ṁ(kg/s)Q̇c(kW)Pcp(kW)COPηex
R134a136.0178.342.296.1071089258.34.2160.305
R245fa152.0195.943.925.4641070240.04.4600.322
R236fa109.3141.832.507.5591078247.04.3630.315
R152a225.4294.168.713.6851084253.24.2800.309
R600a257.1333.376.123.2311077245.94.3790.316
R600289.8373.984.142.8661072241.24.4440.321
RE170328.5426.998.402.5281079248.84.3380.313
R515b121.6159.037.406.8311086255.54.2510.307
R1234ze(E)125.9164.238.296.5971083252.64.2880.310
R1233zd(E)155.7200.444.625.3351069238.04.4910.325

Table 3.

The performances of the heat pump, for different refrigerants.

Note: Operating parameters, same for all refrigerants:

Condensation temperature: 90 °C.

Vaporization temperature: 25°C.

Superheating at the compressor suction (tsh): 10°.

Thermodynamic mean temperature of the geothermal water: 40.55°C.

Thermodynamic mean temperature of the thermal agent: 64.55°C.

Heat flow taken from the evaporator: 830.60 kW.

Efficiency of the reference Carnot cycle: 13.84.

Figure 8.

The COP of the heat pump installation, for different agents (source: results from Table 3).

All refrigerants that have been analyzed are in the category of those that do not affect the ozone layer, with zero ODP potential. In terms of performance, R245fa, R600 and R1233ze(E) refrigerants for which energy efficiency (COP) has the highest values are noted (Figure 8).

R245fa is a colorless, one-component fluid of class HFC (1,1,1,3,3-Pentafluoropropane, C3H3F5) which may replace the use of HCFC R123 and R11. R245fa is among the HFC refrigerants that do not deplete the ozone layer (ODP = 0) but have a significant global warming potential (GWP = 1030). R245fa is used primarily as a blowing and insulation agent for plastic foam insulation, but also as an industrial air conditioning refrigerant, heat recovery systems and high energy recovery systems. HFC-245fa is listed as non-toxic and non-flammable, but exposure to high levels of R245fa can lead to heart sensitization and eye irritation. It falls into safety class B1 [15].

R600 refrigerant is a natural refrigerant of class HC (butane, C4H10) suitable for many refrigeration applications. R600 refrigerant is environmentally friendly and efficient. R600 is a commonly used refrigerant with low environmental impact and good thermodynamic performance. The R600 has low power consumption in refrigeration applications and is compatible with many different lubricants. The R600 is suitable for a variety of tasks in industrial, commercial, and household refrigeration, such as kitchen refrigerators and professional freezers and desks, as well as refrigeration and freezing appliances. R600 is classified as a highly flammable class A3 refrigerant and is subject to strict regulations regarding the amount of agent charged into installations [16].

R1233zd(E) refrigerant is a recent agent, chemically a hydrochlorofluoroolefin (HFO, CF3-CH=CClH). Although this refrigerant is an HCFC and therefore carries chlorine that affects the ozone layer, it is not on the list of fluorinated greenhouse gases that will be eliminated, because the life in the atmosphere is very short (26 days). The properties of this agent are close to the characteristics of the ideal refrigerant: adequate operating pressures, zero GWP potential, zero ODP potential, non-flammable, non-toxic, and adequate volumetric capacity. R1233ze(E) is classified as safety class A1 [15].

Based on the energy performance and physical properties of the selected agents, it was decided that the most suitable agent, to be used in the studied heat pump, is R1233ze(E) refrigerant.

For the refrigerant considered the most suitable, the way in which the energy efficiency (COP) and the heat flow produced at the condenser depend on the subcooling temperature and on the superheating of the vapors at the compressor suction, respectively, was studied. The results are shown in the Figure 9.

Figure 9.

Modification of the coefficient of performance and of the thermal flux produced at the condenser, depending on the subcooling temperature and the degree of overheating (source: simulation in EES software by authors).

It is found that the greatest influence upon the heat pump performances has the subcooling degree of the condensate, while the influence of the degree of overheating at the compressor suction is practically insignificant. For this reason, the minimum value of 10 degrees was considered for the degree of overheating, which ensures that the state corresponding to the compressor discharge does not enter in the wet area and was investigated the way in which only the degree of subcooling influences the performance of the heat pump. The results are shown in Figure 10. It is found that the lower the cooling temperature, the higher the specific cooling power and the lower the required refrigerant flow. As a result, the power required to drive the compressor is reduced. Although the specific thermal load of the condenser increases, the decrease of the refrigerant flow has a stronger influence and as a result the heat flow yielded at the condenser decreases, causing a slight decrease of the heat flow provided in the district heating network. The decrease in the drive power of the compressor has a greater influence on the energy balance, so that overall, the coefficient of performance increases.

Figure 10.

Modification of the thermal flux produced at the condenser, the cooling power, the compressor driving power and the flow of the hot agent, depending on the subcooling temperature, for vapors superheating of 10 degrees (source: simulation in EES software by authors).

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4. The coverage of the HD system

The city of Călimănești is part of the climate zone III of Romania, for which the conventional outdoor temperature for the cold season is tec=18°C [16]. For residential and tertiary buildings, the conventional interior design temperature may be considered tic=+20°C, as [17]. In these conditions, considering the usual thermal characteristics of the buildings and the need of DHW, the maximum thermal load of the DH system of the Călimănești city was evaluated as follow:

maximum thermal load for heating: Q̇heatmax=3500 kW;maximum thermal load for DHW preparation: Q̇dhwmax=500 kW.

Assuming that the heat transfer characteristics are unchanged in relation with outdoor temperature, the thermal load of the DH system, which must be performed by the geothermal station, varies linearly with the temperature of the external environment te [°C], according to the relation:

Q̇GTS=Q̇heatmaxtictetictec+Q̇dhwmaxkWforte10°CE13

The variation of the thermal load in relation to the outdoor temperature is shown in Figure 11. It is considered that the district heating system is put into operation when the mean daily temperature of the external environment is lower than 10°C, producing thermal agent both for the preparation of domestic hot water and for heating. For mean daily outdoor temperatures above 10°C, the system prepares agent for domestic hot water only.

Figure 11.

The thermal load of district heating system in relation with outdoor temperature according to Eq. (13).

In the cold season 2020-2021, the variation of the mean daily outdoor temperature and necessary thermal load for heating system in the Călimănești zone, is shown in the Figure 12 [18]. In recent years, due to global warming, mean temperatures during the cold season have been higher than usual. It can be noted that mean outdoor temperatures below 0°C was recorded in the area for a few days, only in the second half of January and in February, the thermal load during this period being about 70% of the maximum load calculated, based on the conventional temperatures. The system was started on Octomber 01, 2020 and was stopped on April 30, 2021.

Figure 12.

Variation of mean daily outdoor temperature in area and thermal load, in the cold season of 2020-2021 [18].

The operation diagram of the geothermal station coupled with the heat pump is shown in the Figure 13. By cooling the geothermal water with t1=45°C (from 95–50°C) the heat flow introduced in the system is:

Figure 13.

Operation diagram of the geothermal station (source: the drawing of authors).

Q̇he=ṁgwcwt1=ṁhecwtkWE14

and by recovering the residual heat from the geothermal water, the heat flow introduced by the heat pump into the system is:

Q̇hp=ṁgwcwt2COPCOP1=ṁhpcwtkWE15

where, t2=20°C (from 50–30°C) is the degree of geothermal water cooling in the heat pump evaporator, COP is energy efficiency, and t=40°C (from 85–45°C) the temperature difference between the outlet and the inlet of the thermal agent sent into the district heating system.

The two expressions determine the total heat flow that the geothermal station coupled with the heat pump, introduces into the district heating system:

Q̇GTS=ṁgwcwt1+t2COPCOP1=ṁhe+ṁhpcwtkWE16

For the maximum available flow of geothermal water of 10 l/s, the maximum heat flow delivered to the DH system, according to relation (16), is Q̇GTSmax=2889 kW of which 1820 kW directly from geothermal water and 1069 kW from the heat recovered by means of the heat pump. For the operating conditions of the heating system, the energy efficiency of the heat pump was considered COP = 4.5, according to cycle analyze. The external limit temperature up to which the system can operate only with the thermal energy produced in the geothermal water heat exchanger is:

tegw=ticQ̇hemaxQ̇dhwmaxQ̇heatmaxtictec°CE17

and the external limit temperature up to which the system can operate coupled with heat pump is:

tegw+hp=ticQ̇hemax+Q̇hpmaxQ̇dhwmaxQ̇heatmaxtictec°CE18

According to the operating conditions of the heating system, for the limit temperatures specified by relations (16) and (17) the following values resulted for the limit temperatures: tegw=5.7°C and tegw+hp=5.5°C . For outdoor temperature lower than tegw+hp, gas hot water boilers must also be started. The Figure 14 shows how the geothermal station can cover the thermal load of the district heating system, depending on the level of the outside temperature.

Figure 14.

The thermal load coverage in relation with outdoor temperature (source: the drawing of authors).

The adjustment of the geothermal station functioning, so that the heat flow produced can cover the thermal load determined by the outdoor temperature, can be done by changing the flow of the thermal agent sent in the network, with the constant maintenance of its temperature (quantitative regulation) or with the constant maintenance of the flow of the agent sent in the network and the change of its temperature (qualitative regulation). Only quantitative adjustment has been considered in this discussion.

The heat flow required for the DH system needs a water flow to be taken from geothermal borehole:

if te10°C (production of DHW only)

ṁgw=Q̇dhwmaxcwt1kg/sE19

if 10°C>te5.7°C (production of DHW and heating with geothermal water only)

ṁgw=Q̇heatmaxtictetictec+Q̇dhwmaxcwt1kg/sE20

if 5.7°C>te5.5°C (production of DHW and heating with geothermal water and heat pump)

ṁgw=Q̇heatmaxtictetictec+Q̇dhwmaxcwt1+t2COPCOP1kg/sE21

The HD system can cover the required thermal load using only geothermal water, up the outdoor temperature of +5.7°C, when the maximum geothermal water flow of 10 l/s (9.62 kg/s) is extracted from the borehole. Below this temperature, the DH system coupled to the heat pump can covers the required thermal load up to temperature of −5.5°C, when the flow of extracted geothermal water is also maximum. For lower outdoor temperatures, the thermal load is supplemented by the commissioning of hot water boilers with gaseous fuel.

If the temperature of the thermal agent sent to the district heating system is kept constant, its flow rate depending on the temperature of the external environment is expressed:

ṁta=ṁhe+ṁhp=Q̇heatmaxtictetictec+Q̇dhwmaxcwtkg/sE22

The variation of these flows in relation to the outdoor temperature is shown in the Figure 15.

Figure 15.

The geothermal water flow and the thermal agent flow in relation with outdoor temperature, according to Eq. (19)-(22).

Coupling the geothermal station with a heat pump to recover the thermal energy of the wastewater discharged from the geothermal heat exchangers, allows to increase the flow of thermal agent introduced into the district heating system and cover about 70% of its maximum thermal load. Under these conditions, the district heating system can ensure the thermal comfort of consumers only up to an outside temperature of around −6°C (Figure 16).

Figure 16.

The thermal load coverage in relation with outdoor temperature (source: the drawing of authors).

Examining the way in which the climatic conditions in the area have manifested in recent years, by coupling the geothermal station with a heat pump, the maximum available flow of geothermal water can ensure the coverage of the entire thermal load of the heating system, without the use of gas-fired hot water boilers. Due to the global warming phenomenon, the mean daily temperatures during the cold season were higher than the usual temperatures for this period.

As can be seen in the Figure 16, according to the weather archive in the area, during the cold period of the 2020-2021 season, in just a few days the average outdoor temperature dropped below −5°C, the temperatures in the rest of this period being in the range 0 ... + 5°C. Under these conditions, the thermal load of the system can be covered throughout the heating season without the need to come into operation the hot water boilers with gaseous fuel.

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5. Conclusions

  • The solution of using medium enthalpy geothermal water, from the deposits located in the lower basin of the Olt River, as a heat source for the district heating system of Călimănești is part of the current concerns to reduce the consumption of conventional fuels, generated both by depletion of reserves and the need to reduce greenhouse gas emissions. At the same time, the geothermal energy being provided free of charge by nature, continuously and renewable, allows the creation of heating systems that provide the population with heat at very low prices.

  • Concerns both to increase energy efficiency and to protect reserves, have required finding solutions that allow to obtain a maximum amount of energy from available resources. In this context, the implementation in the district heating system of Călimănești city of a heat pump is a modern and very good solution for total capitalization of the thermal potential of the geothermal water available from drilling 1009. Firstly, the heat pump allows the full use of the thermal potential of geothermal water, which can be discharged at a temperature close to the environment, and secondly, increases the flow of heat sent to the district heating system, allowing either its expansion or the connection of new consumers to the network.

  • The available flow of geothermal drilling water can continuously provide the necessary thermal energy for the preparation of domestic hot water. The geothermal water flow required for this purpose represents about 25% of the available well flow. During the period when the district heating system is not working, the thermal energy necessary for the preparation of domestic water can be provided entirely from geothermal water.

  • During the cold season, when the district heating system comes into operation, the maximum available geothermal water flow, can only provide the thermal energy needed to prepare the domestic water and about 40% of the thermal energy needed to cover the maximum heating load of the system. Under these conditions, the district heating system can ensure the thermal comfort of consumers only up to an outside temperature of around +5°C. If the outside temperature drops below this limit, it is necessary to start the gas-fired hot water boilers. The implementation of a heat pump, which recovers the thermal potential of the wastewater discharged from the heat exchangers of the geothermal station, allows to increase the capacity of the system by about 60%. Under these conditions, given the mild climate in the area in recent years, this solution would eliminate the need to use hot water boilers with gaseous fuel, which are kept covering peak loads or possible damage.

  • The water extracted from the aquifer of the area contains a large quantity of combustible gases, with a content of over 90% methane. This feature is favorable for the implementation of a gas cogeneration system, which would also provide the electricity needed for the operation of the heat pump.

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Written By

Laurentiu Constantin Lipan and Sorin Dimitriu

Submitted: July 19th, 2022 Reviewed: August 19th, 2022 Published: November 25th, 2022