Smart Combined Heat and Power System, Using Geothermal Low Enthalpy Resources

Abstract

Romania is among the European countries that have rich geothermal water resources. Unfortunately, these resources are of medium and low enthalpy, which makes their uses only direct, for space heating, spa treatments, greenhouse heating and other applications in agriculture and fish farming. However, recent regulations regarding the reduction of greenhouse gas emissions and fossil fuel consumption have led to increased research into the possibility of using these low-enthalpy resources for electricity production using ORC technology. The authors present a case study regarding the generation of electricity through ORC technology, using geothermal water with a temperature of 75°C from a geothermal reservoir located in the north of Bucharest, in the perimeter of Moara Vlăsiei locality. The performance of the installation is studied in relation to the working agents that may be used and the cooling medium available. The optimal operating conditions, for which the maximum electrical power is obtained, are also determined. The study is timely, considering the metropolitan projects to realize in the area a heating system using geothermal water and which can have high energy efficiency through its smart combination with electricity production.

1. Introduction

Since the dawn of human existence, water has been considered as the primordial element, “Fons et Origo”, being revered as the matrix of all possibilities. From the beginning, our ancestors considered water as the foundation of all cosmic manifestation, the receptacle of all forms and all germs, the waters symbolizing the primordial substance from which all forms are born and into which they all return, by regression or cataclysm. That is why, since ancient times, thermal springs have been shrouded in mysticism, being revered both as a source of health and miraculous powers, and as a source of heat. The need for warmth arose with the beginning of human existence, primarily to create shelters to provide protection against cold periods. This need was initially met by using natural resources, such as the heat of the ground or that provided by thermal waters. Through the discovery of fire, mankind learned that heat can be obtained voluntarily, through combustion, marking the beginning of the era of using fossil or renewable fuels. The ease of producing heat by burning these resources and at the same time their high availability, however, overshadowed throughout history the interest in using the heat provided by natural geothermal resources.

However, archeological findings have shown that natural geothermal springs have continued to be used for heating and thermal baths throughout time. The earliest uses of geothermal energy have been found to have occurred over 10,000 years ago in North America, where the inhabitants used the hot springs in the various areas for both practical and spiritual purposes, being believed to be endowed by the gods with magical powers of healing [1]. Evidence of these uses has also been found in the ancient Greeks as well as the Romans. Sites dating as far back as the first century (AD) have been discovered, attesting to the use of geothermal energy for heating homes, hot baths and culinary activities, but uses in this regard were limited to locations where hot geothermal water was available naturally in the form of hot springs [2]. The Greek physician Hippocrates (460–320 BC) promoted and highlighted the health benefits of hot baths, and the Roman scholar Pliny the Elder (23–79 AD) wrote about the special benefits of hot mineral baths for people suffering of muscles, joints or paralysis [3].

The discovery and use of fire was a first and huge step towards civilization, but it took approx. a million years for mankind to take a second giant step, the discovery and use of electricity. The first type of electricity, static electricity, was discovered by the ancient Greeks around 600 BC, when they found that rubbing a piece of amber produced an attraction force on the hairs of an animal’s fur, but it was not until the sixteenth century that the first generator of static electricity was invented and more types of practical uses appeared. With Alessandro Volta in the early 1800s, electricity became a constant source of energy and the first reliable source capable of continuously producing electricity appeared. From this moment, it was only a step to the other epoch-making inventions that opened the way for the huge technical-scientific development of this field. Today’s world is unimaginable without electricity; there is no level of humanity in which electricity does not play one of the main roles.

The eighteenth to twentieth centuries represented the period in which the invention and development of thermal power machines and electric generators allowed the thermal energy produced by burning fuels to be commercially transformed, on a global scale, into electrical energy, continuously and in quantities that would satisfy all consumption requirements. The most affordable and readily available fuels used in power plants are coal, fuels derived from crude oil and natural gas. Although these fossil fuels have been discovered since ancient times, the production of fuels derived from crude oil only began in the eighteenth and nineteenth centuries, when the first refineries were built and the exponential development of the techno-scientific revolution began, which led mankind to the current level of knowledge and technology. The intense and rapid development of means of transport and industrial activities, as well as the huge increase in the world’s population, has made the need for energy ever greater, leading to the burning of ever increasing an amount of coal, gas and petroleum fuels, especially for transport and power generation. All these burning processes led to the generation of huge amounts of carbon dioxide which, combined with the reduction of regeneration capacity due to massive deforestation, determined the alteration of the natural balance of carbon dioxide and caused the emergence of the dangerous phenomenon of global warming. This situation, generating of strange climate changes we are facing today, combined with the depletion of fossil fuel reserves, has led humanity to concern more and more about finding solutions to increase energy efficiency and find sources of alternative energies, that reduce greenhouse gas emissions and in the same time, to meet global energy needs. Based on these considerations, amplified by the oil and gas crisis, the economic recession, as well as various conflicts and international events, the exploitation of geothermal energy resources for the production of heat and electricity has become a particularly promising alternative [4].

The Earth is estimated to have an internal heat content of 10³¹ joules (3∙10¹⁵ TWh), about 20% of which is waste heat from planetary accretion; the rest being attributed to past and present radioactive decay of natural isotopes [5]. Geothermal energy is defined as thermal energy that can be obtained in profitable conditions, from internal heat of the Earth, from hot waters or rocks located at depths up to 3000–5000 m below the earth’s crust.

Geothermal systems can be classified according to the temperature level into three categories: high enthalpy systems (above 180°C), medium enthalpy systems (100–180°C) and low enthalpy systems (below 100°C).

High enthalpy systems consist of hot rocks generally located at depths greater than 3500 m and are due to thermal anomalies in the Earth’s mantle or volcanic activity. Medium and low enthalpy systems are systems made up of hot aquifers located at depths of several hundred meters, up to approx. 3000 m and which is due to the circulation of water between the accumulation zone and greater depths with high temperatures [6].

Geothermal energy can be used directly as a heat source for space heating needs, spa treatments, agriculture and fish farming or indirectly for electricity production.

Currently, according to country reports, geothermal energy is used directly in at least 88 countries. It is estimated that the thermal power 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). The fields of use are according to the reported data: geothermal heat pumps (58.8%), swimming pools and thermal baths (18%), direct heating of residential and tertiary spaces (16%), heating of vegetable greenhouses (3.5%), uses in various industrial processes (1.6%), aquaculture and fish farming (1.3%), grain drying (0.35%), other uses (0.45%). The highest amounts of geothermal energy consumed were reported in order in China, USA, Sweden, Turkey and Japan [7, 8].

The use of geothermal energy for electricity production began with the exploitation of the resources provided by high-temperature geothermal systems, above 150°C. These are constituted by natural sources of dry steam or hot water with high pressure that can be vaporized by expansion (flash-steam). The world’s first plant to produce electricity using dry geothermal steam was built in Larderello, Italy in 1904, producing enough power to light a few electric bulbs. In 1915, the plant reached a power of 5MW, and currently it is 545 MW [4].

After the Second World War, several geothermal plants with dry steam or flash steam were put into operation: Wairakei—New Zealand (1958–1960, 98 MW), Pathe—Mexico (1959, 3.5 MW) and Geysers—USA (1960, 11 MW). In 1970, research began at Las Alamos, USA, on the production of steam by vaporizing water injected into hot fractured rocks from deep. The project managed to achieve a power of 5 MW, but was stopped due to the generation of seismic movements as a result of the fracturing process. Starting from the results obtained here, it was possible after the 90s to obtain electrical energy on the same principle in other countries: France (Soultz sous Forêt, 1.5 MW), Germany, Japan and Great Britain [9].

High-temperature geothermal resources represent only a fraction of the world’s geothermal resources. Many countries do not have high enthalpy resources on their territory suitable for electricity generation, but only resources of medium enthalpy and in most cases of low enthalpy. Exploiting them for electricity production became possible only after 1950, thanks to the development of ORC and Kalina technologies [10]. The comparative thermodynamic analysis between the ORC and Kalina systems has shown that the ORC system has a better profitability and can also use low enthalpy sources, with temperature levels up to 70–80°C. Basically, the operating principle of the ORC system is the same as the thermodynamic Rankine cycle, the difference being that instead of water, organic fluids are used, which for the pressures required to operate turbines or expanders, the saturation temperatures are much lower than water. The first ORC plants using low-enthalpy geothermal water, built as pilot plants, were built in 1952 at Kiabukwa (Democratic Republic of Congo, 200 kW, geothermal water temperature 91°C) and in 1967 at Paratunka (Kamchatka—USSR, 670 kW, temperature geothermal water 85°C) [11].

Although these first installations have demonstrated that it is possible to produce electricity also from low-enthalpy geothermal resources, using ORC technology, economic profitability can only be achieved if the condensation temperature of the working agent can be maintained at a level down. In 2006, a power plant with two groups of 200 kW each was completed in Chena, Alaska, using water from the thermal springs in the area, with a temperature of 74°C. The plant operates continuously, greatly reducing the cost of electricity and eliminating the previously used liquid fuel (diesel). The plant can be cooled with air or water, depending on the season, and it operates in profitable conditions due to the cold climate, the average annual temperature being −5°C [12]. Also in 2006, a consortium of nine European organizations started a project, LOW-BIN, which aimed to build two ORC installations to produce electricity from low-enthalpy geothermal waters. The project was realized in 2009 by building a 7 MW power plant in Simbach am Inn, Germany, using geothermal water with a temperature of 80°C and by building a 50 kW power plant in Oradea, Romania, using geothermal water with a temperature of 104°C. Both installations were integrated into the local heating system. Both plants operated for a while, reaching design parameters, but were shut down for economic reasons [13, 14].

At the present time, the concern of the entire world is to meet the ever-increasing demand for electricity and the reduction of greenhouse gas emissions resulting from the burning of fossil fuels. The current challenges are to generate electricity from renewable resources, friendly to the environment and generating low carbon dioxide emissions. Low-enthalpy geothermal energy sources satisfy these wants. They are huge, have not been used extensively before and can be used to meet future electricity demand, being sustainable for generations [15]. Many countries have completed or have ongoing projects to use low-enthalpy geothermal energy sources for the production of electricity both for rural electrification and for large units connected to local electricity networks: USA (Chena Project, Alaska, 400 kW), Indonesia (Sarulla Project, Sumatra, 330 MW), Turkey (Atça Project, Denizli, 32.8 MW). In Greece, rich in low-enthalpy geothermal resources, it is expected to obtain a production of approx. 45.43 GWh/year (10% of electricity demand) by exploiting such resources [16].

2. Possibility of implementing a LOW-BIN type project in the Moara Vlăsiei geothermal area, in the north of Bucharest

The geothermal reservoirs from the north of Bucharest (Figure 1) are located at a depth range between 1800 and 3200 m. They have a surface determined to date of approx. 300 km², and the geothermal gradient is between 28…34°C/1 km. The water temperatures obtained through research boreholes are in the range of 58…85°C at the wellhead (the lowest being near Bucharest and the highest near Snagov), and the flow rates are 22…35 l/s. Due to the rather high content of salts (1.5…2.2 g/l) and the high content of hydrogen sulphide (up to 30 ppm), reinjection of waste water into the deposit is mandatory for environmental protection. The geothermal reservoir in the north of Bucharest is not of the artesian type (no pressure in the reservoir) and as a result the extraction must be carried out with the help of bottom pumps, the water level in the wells being approx. 80 mbgl (meters below ground level). To create a pilot station and study its energy efficiency, a borehole that is currently under conservation in the perimeter of the Moara Vlăsiei commune can be used. The borehole has a depth of 3000 m and can provide a constant water flow of 35 l/s, with a wellhead temperature of 75°C, only needing to open it and bring it into working condition.

Figure 1.

The geothermal reservoirs in the north of Bucharest (source: Authors’ drawing).

The commune of Moara Vlăsiei belongs to an area with an average natural hydrographic network, represented by several streams known as Cociovaliștea and Vlăsia. Transformed by the Ialomița River into a fluvial estuary through the alluviation of the valley, the Căldăruşani Lake was transformed from the union of the Cociovaliștea stream with the Vlăsia stream, both springing from the plain. The depth of the lake registers higher values than those of the Ialomița River. With an area of 224 ha (only clean water, without reeds), Caldărușani Lake has an average depth of 4.5 m and a maximum of 9–10 m. The lake, with fresh water and with a mineralization of 521 mg/l (NaHCO₃ type), allows visibility up to a depth of approximately 2.5 m. The waves, with a height of 50…80 cm, are formed by the wind blowing at a speed of 34.8 m/sec. Near Grădiștea commune, Căldărușani Lake stretches for a distance of 4 km, of which 1.5 km is clear water, the rest being covered by reeds. The Moara Vlăsiei water reservoir is located on the Cociovaliștea valley, from which it is fed, benefiting from the oxygenated water of the river. In the north of the commune, along it, a series of lakes is formed, of which Moara Vlăsiei 2 pond is the most famous. It has a total area of 28 ha, being divided into 2 basins: the carp pond, which is 17 ha, and the area called “wild”, with an area of 11 ha. The total length of the pond is about 2 km. The water depth varies between 2 and 4 meters and the bottom is generally silty and there are no submerged trees.

The climate of the area is temperate - continental, with a few days a year below the freezing point, the ponds that are part of the hydrographic system of the Cociovaliștea valley rarely freeze only on the surface. In Figure 2 are shown the variations in average daily, minimum and maximum temperatures for the period 01.01.2022– 30.10.2023.

Figure 2.

Temperature variation during the period 2022-01-01…2023-10-27 (source: Meteoblue).

It can be seen that in recent years, during the cold season, only the night temperatures were slightly below the 0°C limit, while the average daily temperature was never below this value. Characteristic of the area is the fact that during the summer, the maximum daytime temperature of the atmospheric air was within the limits of 30…40°C. This finding is particularly important for the operation of an ORC geothermal plant, because the high temperature of the atmospheric air greatly raises the condensing temperature of the working agent, causing the energy efficiency to decrease drastically. In the conditions where the available geothermal water has a wellhead temperature of 75°C, to obtain a reasonable efficiency of 5…7%, water cooling of the condensers would be favorable. This, however, requires the authorized design of the adduction and the obtaining of approvals from the Romanian National Water Authority (ANAR), respectively the National Environmental Protection Agency (ANPM) for the use of water from the Cociovaliștea hydrographic basin, when necessary, considering that the lake area makes part of the “Nature 2000” protected area ROSPA0044.

Currently, the General Urban Plan (PUG) of the Moara Vlăsiei commune does not make any reference to the possibility or intention of exploiting the geothermal resources located within the commune’s perimeter. As a result, the implementation of a LOW-BIN type pilot plant cannot be coupled with the possibility of harnessing the thermal energy of the water discharged from the plant, which must be re-injected back into the geothermal reservoir. The content of salts and hydrogen sulphide of the water do not allow it to be cooled and discharged into the hydrographic basin of the Cociovaliștea River. Considering that the area is of tourist interest for fishing, ornithological observations, boat walks on the lake and the Căldărușani monastery located in the immediate vicinity, the LOW-BIN type installation for the production of electricity, through the available hot water, could generate in the future the development of relaxation units (thermal baths) or create the prospect of centralized heat supply to the residential sector or public institutions in the area. In this sense, we can cite the Metropolitan Project no. 13 “Utilization of geothermal resources from the Bucharest-Ilfov Region for heating homes” of the Bucharest City Hall, Ilfov County Council, which has as objectives:

• Reduction of CO₂ emissions resulting from the production of thermal energy from conventional sources, by capitalizing on the geothermal resources in the northern part of the Bucharest-Ilfov Region;

• The use of geothermal resources in the Bucharest-Ilfov Region as a source of renewable and non-polluting energy for heating homes in the Territorial Administrative Units (UATs) located in the vicinity of thermal water sources;

• Reduction of home heating costs borne by county citizens;

• Increasing the county’s energy independence from natural gas imports.

The estimated budget of the project is EUR 100 million (including the digging of 4 wells of EUR 5 million each), the period of the works being estimated between 2023 and 2027 [17].

3. Analysis of plant operation and performance prediction

The condenser of the plant can be cooled with atmospheric air or with water.

Water cooling is more favorable because it provides low condensing temperatures and consequently a high expander expansion ratio, while atmospheric air cooling involves higher condensing temperatures and consequently a lower expander expansion ratio. Water cooling is simpler and requires heat exchangers reduced in size and heat transfer surface. Water cooling, however, involves ensuring the necessary flow rates, which most of the time involves high expenses for deep drilling or the arrangement of adductions, if surface water is used. To these must be added the energy required for the pumps and the approvals from the competent authorities.

Air cooling involves larger heat transfer surfaces and as a result larger dimensions for the heat exchangers, but in return does not require installation costs and environmental approvals. Air cooling, however, is strongly influenced by the temperature of the atmospheric air, which means that during the cold season the operation of the installation is very efficient (maximum power obtained) the condensation temperature of the working agent being low, but during the warm season, as it increases atmospheric air temperature, the condensation temperature of the working agent in the installation increases, which drastically reduces the power obtained at the electric generator. However, this can be improved by using evaporative coolers in the warm season, which combine air cooling with water cooling. The required cooling water being used in a closed circuit, leads to very low consumption that can be provided without problems from local sources. In any case, the organic fluid must condense in the pipes that form the heat transfer surface of the coolers, so that the temperature difference between the condensing agent and the temperature of the cooling air is as minimal as possible, in order to ensure the maximum efficiency of the installation (obtaining a power maximum possible at the generator). There are several types of air-cooled condensers suitable for use in ORC installations using atmospheric air cooling: dry coolers; evaporative coolers that work in wet mode; hybrid coolers that can work both in dry and wet mode; coolers with adiabatic air cooling (adiabatic coolers).

From the point of view of operation, the scheme of the installation and the way of temperatures variations in its evaporator and condenser are identical, the difference consisting in the variation range of the temperature of the coolant, which determines the level of the condensing temperature. During the cold period, the operation of the installation becomes very efficient because the power obtained from the generator increases, the condensation temperature can being reduced as the temperature of the atmospheric air decreases. In warm periods, the use of condensers with evaporative, hybrid or adiabatic cooling allows maintaining the condensing temperature at a level at which the power obtained at the generator is high enough, with a reduced consumption of cooling water. Compared to water cooling, which simplifies the construction of the module, but involves large investments for the arrangement of water intake and discharge, as well as approvals for construction and operation, air cooling leads to good results, the investment being limited only to the purchase of the equipment (the heat exchangers) and ensuring a source of water to supplement the evaporated quantity during the periods when working in a wet regime.

The operation scheme of the module is shown in Figure 3. The vapors generator of the installation consists of two sections. The hot geothermal water from the extraction well first enters in the vaporizer itself, where the saturated liquid with state 5 is vaporized, and obtaining superheated vapors with state 1. In the second part of the vapor’s generator, the working agent is preheated to the saturation temperature. The temperature to which the geothermal water can be cooled depends on the need to achieve a minimum temperature difference allowed in point 5, at the exit from the preheater (pinch-point), within 3…5 degrees, this requirement obviously determining the vaporization temperature. The condensing temperature is determined by the temperature of the available coolant, the allowable temperature variation for the cooling medium and the minimum allowable temperature difference at the outlet of the condenser. To avoid the operation of the expander in the wet area, a slight overheating of about 5 degrees of the vapors at the exit from the vaporizer was considered. Since the vapor state at the exit of the expander is very close to the saturation state, it is not necessary to introduce a regenerative heat exchanger to take the superheating heat, in order to preheat the working agent before entering the vaporizer system. The expansion process in expander was considered adiabatic, irreversible, the internal efficiency of the expander being set to a usual value for turbo type expanders. State 3 of the liquid leaving the condenser was considered saturated liquid state, and the process of raising the pressure in the steam generator feed pump was also considered an adiabatic, irreversible process.

Figure 3.

Scheme of operation of the installation (source: Authors’ drawing).

To simulate the operation of the plant under different conditions, the vaporization temperature ${\mathit{t}}_{\mathit{v}}$ was considered as the independent variable. The temperature ${\mathit{t}}_{\mathbf{3}\mathit{ac}}$ to which the geothermal water can be cooled is determined from the heat balance equation of the vaporizer, respectively of the preheater:

$$t_{3\mathit{ac}}=t_{2\mathit{ac}}-\left(t_{1\mathit{ac}}-t_{2\mathit{ac}}\right)\frac{h_5-h_4}{h_1-h_5}\left[{}^{°}C\right]$$E1

in which the temperature of the geothermal water at the outlet of the vaporizer is set so that a temperature difference (pinch-point) $∆t_{\mathit{pp}}=3\dots 5^{°}C$ is achieved here (Figure 4, left):

Figure 4.

Temperature variation in evaporator and condenser (source: Authors’ drawing).

$$t_{2\mathit{ac}}=t_v+∆t_{\mathit{pp}}\left[{}^{°}C\right]$$E2

The condensation temperature depends on the initial temperature $t_{\mathit{ar}1}$ (or $t_{a1}$) of the available coolant, on its temperature variation $∆t_{\mathit{ar}}$ and on the minimum admissible temperature difference at the exit from the heat exchanger (Figure 4 right):

$$t_c=t_{1\mathit{ar}}+∆t_{\mathit{ar}}+∆t_{\mathit{pp}}\left[{}^{°}C\right]$$E3

The study of the installation performances, starting from the flow rate and temperature of the available geothermal water, was carried out in two variants: water cooling and air cooling.

According to the operation scheme shown in Figure 3, the operating conditions of the plant were considered as Table 1.

The considered parameter	Value	Unit
Geothermal water temperature at the wellhead	75	°C
Flow of geothermal water	35	l/s
Coolant temperature (depending on the season)	0…30	°C
Minimum temperature difference in the heat exchangers (pinch-point)	3	deg
Overheating of vapors	5	deg
Variation of the coolant temperature	5	deg
The isentropic efficiency of the expander	0.85
The isentropic efficiency of the circulation pumps	0.75
Vaporization temperature	40…60	°C
Condensation temperature (depending on the coolant available)	5…25	°C

Table 1.

The operating conditions of the plant.

Figure 5 shows the thermodynamic cycle of the plant, represented in temperature-entropy (T − s) diagram, highlighting the energy flows that the refrigerant exchanges with external environment.

Figure 5.

Thermodynamic cycle in T − s diagram (source: generated in EES software).

The state parameters at the characteristic points of the cycle were calculated according to the algorithm presented in Table 2, using the Engineering Equation Solver (EES) calculation environment.

State point	Description	Known state parameters	Functions EES for state parameters
1	Superheated vapors at the inlet of the expander	$p_1=p_v$; $t_1=t_v+∆t_{\mathit{si}}$	$h_1=h\left(p_v,t_1\right)$; $s_1=s\left(p_v,t_1\right)$
2s	The end of the isentropic expansion process	$p_{2s}=p_c$; $s_{2s}=s_1$	$t_{2s}=t\left(p_c,s_{2s}\right)$; $h_{2s}=h\left(p_c,s_{2s}\right)$
2	The end of the actual expansion process	$p_2=p_c$; $h_2=h_1-{\eta }_{\mathit{exp}}\left(h_1-h_{2s}\right)$	$t_2=t\left(p_c,h_2\right)$; $s_2=s\left(p_c,h_2\right)$
3vs	Dry saturated vapors after cooling	$p_{3\mathit{vs}}=p_c$; $x_{3\mathit{vs}}=1$; $s_{2s}=s_1$	$h_{3\mathit{vs}}=h\left(p_c,x=1\right)$; $s_{3\mathit{vs}}=s\left(p_c,x=1\right)$
3	Saturated liquid at the outlet of the condenser	$p_3=p_c$; $x_3=0$; $t_3=t_c$	$h_3=h\left(p_c,x=0\right)$; $s_3=s\left(p_c,x=0\right)$
4s	The end of the isentropic pumping process	$p_1=p_v$; $s_{4s}=s_3$	$t_{4s}=t\left(p_v,s_{4s}\right)$; $h_{4s}=h\left(p_v,s_{4s}\right)$
4	The end of the actual pumping process	$p_4=p_v$; $h_4=h_3+\left(h_{4s}-h_3\right)/{\eta }_{\mathit{pp}}$	$t_4=t\left(p_v,h_4\right)$; $s_4=s\left(p_v,h_4\right)$
5	Saturated liquid at the outlet of the preheater	$p_5=p_v$; $x_5=0$; $t_5=t_v$	$h_5=h\left(p_v,x=0\right)$; $s_5=s\left(p_v,x=0\right)$
1vs	Dry saturated vapors	$p_{1\mathit{vs}}=p_v$; $x_{1\mathit{vs}}=1$; $t_{1\mathit{vs}}=t_v$	$h_{1\mathit{vs}}=h\left(p_v,x=1\right)$; $s_{1\mathit{vs}}=s\left(p_v,x=1\right)$

Table 2.

The calculating algorithm for the values of the state parameters.

The characteristic sizes of the installation are:

• Specific cooling capacity of the condenser:

$$\left|q_c\right|=h_2-h_3\left[\mathrm{kJ}/\mathrm{kg}\right]$$E4

• Specific heat load of the evaporator:

$$q_v=h_1-h_5\left[\mathrm{kJ}/\mathrm{kg}\right]$$E5

• Specific heat load of the preheater:

$$q_{\mathit{pi}}=h_5-h_4\left[\mathrm{kJ}/\mathrm{kg}\right]$$E6

• The specific mechanical work of expansion in the expander:

$$l_{\mathit{exp}}=h_1-h_2\left[\mathrm{kJ}/\mathrm{kg}\right]$$E7

• The specific mechanical work of pumping:

$$\left|l_{\mathit{pp}}\right|=h_4-h_3\left[\mathrm{kJ}/\mathrm{kg}\right]$$E8

• The mass flow of geothermal water has the expression:

$${\dot{m}}_{\mathit{ac}}=\frac{{\dot{V}}_{\mathit{ac}}{\rho }_{\mathit{ac}}}{1000}\left[\mathrm{kg}/\mathrm{s}\right]$$E9

where ${\dot{V}}_{\mathit{ac}}$ [l/s] is the flow rate of the hot geothermal water well, and ${\rho }_{\mathit{ac}}$ [kg/m³] is the density of the hot thermal water.

The flow rate of the working agent results from the thermal energy balance of the vaporizer:

$$\dot{m}={\dot{m}}_{\mathit{ac}}\frac{c_{\mathit{ac}}\left(t_{1\mathit{ac}}-t_{2\mathit{ac}}\right)}{h_1-h_5}\left[\mathrm{kg}/\mathrm{s}\right]$$E10

The heat flow received in the vapors generator from the geothermal water (hot source) is:

$${\dot{Q}}_v={\dot{Q}}_{\mathit{SC}}=\dot{m}\left(q_v+q_{\mathit{si}}\right)\left[\mathrm{kW}\right]$$E11

and the heat flow transferred to the condenser (cold source) has the expression:

$$\left|{\dot{Q}}_c\right|=\left|{\dot{Q}}_{\mathit{SR}}\right|=\dot{m}\left|q_c\right|\left[\mathrm{kW}\right]$$E12

resulting in the expression for the thermal efficiency of the cycle:

$${\eta }_{\mathit{th}}=\left(1-\frac{\left|{\dot{Q}}_{\mathit{SR}}\right|}{{\dot{Q}}_{\mathit{SC}}}\right)\bullet 100\left[\%\right]=\left(1-\frac{\left|q_c\right|}{q_v+q_{\mathit{si}}}\right)\bullet 100\left[\%\right]$$E13

To select the optimal working agent, the operation of the installation was simulated using the EES computing environment, considering several usual agents for ORC installations, considering the variable vaporization temperature in the range of 40…60°C, the coolant temperature being constant and equal to 20°C. The agents taken into account are presented in Table 3. On the one hand, these are agents that have thermodynamic properties that correspond to the use in an ORC installation, under the given conditions, and on the other hand, they are ecological agents, accepted according to “EU Regulation 517/ 2014 on fluorinated greenhouse gases”, having zero ozone depletion potential (ODP) [20].

Refrigerant	Group	Critical temperature; tcr [°C]	ODP	GWP/CO2	Safety group
R123	HCFC	183.7	0.02	9100	B1
R245fa	HFC	154.0	0	858	B1
R134a	HFC	101.0	0	1300	A1
RE170 (DME)	HC	127.2	0	1	A3
R600a	HC	134.9	0	3	A3
R600	HC	154.2	0	4	A3
R1233zd(E)	HFO	165.5	0	1	A1
R1234ze(E)	HFO	109.4	0	6	A2L

Table 3.

Usual working agents (refrigerants) accepted [18, 19].

The simulation results are shown in Figure 6. It can be seen that the performance of the installation, the power of the electric generator and the thermal efficiency depend on how the vaporization temperature is chosen:

• Whatever the working agent used, the maximum power of the installation is obtained if the vaporization temperature is chosen around 50°C;

• Agents leading to maximum performance are found to be R134a (HFC) and R1234ze(E) (HFO).

Figure 6.

Electric generator power and efficiency in relation to the choice of vaporization temperature and refrigerant (source: generated in EES software).

The optimal agent for this installation is considered R134a because it allows obtaining the maximum power (210 kW) for the maximum drilling flow (35 l/s), it has a negligible environmental impact (GWP = 0; ODP = 1300), non-toxic, non-flammable (A1) and is the agent with the greatest use in installations operating after reverse cycles, being easily accessible. Also, on the market there are equipment dedicated to this refrigerant, well designed and tested. The R1234ze(E) refrigerant has quite similar performance but is more flammable and more expensive.

Figure 7 shows the result of the simulation of the plant operation, for the refrigerant R134a, considered the coolant temperature 20°C and the vaporization temperature of 50°C for which the maximum power is obtained.

Figure 7.

The result of the simulation of the ORC plant operation (source: generated in EES software).

An important parameter that determines the performance of the installation is the temperature of the cooling agent. If it comes from a deep well, its temperature is constant with values in the range of 10…15°C. However, if it is surface water (river, lake), its temperature is variable and depends on the season, being within the limits of 5…25°C. If the cooling agent is atmospheric air, its temperature, according to the climate of the area, can be in the range of 0…30°C. Changing the temperature of the cooling agent causes a change in the temperature and, respectively, the condensing pressure of the refrigerant. This primarily results in the modification of the expansion ratio of the expander and as a result of the specific mechanical expansion work. At the same time, the specific heat load of the vaporizer system also changes, which leads to a change in the flow rate of the working agent. If for each value of the cooling agent temperature, the vaporization temperature changes, it is found that the power at the generator changes according to a parabolic law, presenting a maximum for a certain vaporization temperature. As the cooling agent temperature decreases, the power generated by the plant increases, with maximum power being obtained at lower and lower vaporization temperatures. Figure 8 shows, for the chosen agent R134a, the variation of the power at the generator in relation to the change in the vaporization temperature.

Figure 8.

Electric generator power in relation to the choice of vaporization temperature (source: generated in EES software).

For cooling water extracted from deep wells with a temperature of approx. 15°C, the plant can produce about 250 kW. For cooling water from surface storage the power produced can vary between 150 and 350 kW depending on the season and for air cooling 75…400 kW depending on the season also. For the operation of the installation, the cooling water flow must have a fairly large available 150…220 l/s (540…800 m³/h) which could require (depending on local conditions) the use of a closed circuit, with cooling tower. At a temperature of 15°C, the installation operating with a dry condenser (dry cooler) can produce approx. 210 kW. At high temperatures, corresponding to the summer season, the power produced by the generator drops below 100 kW, which does not even cover the energy consumption of the pumps. To keep the power at a reasonable level, at a threshold of 150 kW the condenser must work in the wet evaporating mode. It can be seen that the air flow required to cool the condenser is quite high, being around 250…350 m³/s (0.9∙10⁶…1.3∙10⁶ m³/h).

Figure 9 shows the variation of the vaporization temperature at which the maximum power of the installation is obtained, as well as the temperature at which the used geothermal water is discharged from the installation, depending on the temperature of the available cooling agent. It can be seen that, in order to obtain the maximum power at the electric generator, the temperature and vaporization pressure must be constantly adjusted according to the temperature of the available cooling agent.

Figure 9.

Optimum vaporization temperature and geothermal water outlet temperature depending on the coolant temperature, in maximum power mode (source: generated in EES software).

The lower the temperature of the cooling agent, the more power is obtained from the plant and at the same time the geothermal wastewater is discharged with a lower temperature. The discharge temperature of the waste water is in the range of 40…50°C, depending on the temperature of the cooling agent. At this temperature level, to increase energy efficiency, the wastewater discharged from the plant can be used directly for heating or as a source for a heat pump.

As shown in Figure 10, the efficiency of the installation falls within the limits of 7.5…5% depending on the temperature of the cooling agent, values also reported in the specialized literature for ORC installations that have as heat source low enthalpy geothermal water [21].

Figure 10.

Plant efficiency in relation to the choice of vaporization temperature (source: generated in EES software).

4. Conclusions

In Romania there are important reserves of geothermal water with low enthalpy which are currently only used directly for heating or for relaxation and spa treatment units. This study, as well as others on the same theme, highlights the fact that electricity can also be obtained in the case of these low-enthalpy resources, within the limits of 1500…3000 MWh/year, from geothermal wells with usual flow rates of 30…40 l /s.

The necessary equipment does not raise problems, because the mentioned studies have shown that the most suitable working agent is the classic refrigerant R134a, for which the manufacture of equipment is very well developed. Although this refrigerant is still in use, its replacement in the future does not raise problems because there is developed the refrigerant of the new generation of “green” agents R1234ze (E), which has the same characteristics and allows the same performance.

The main problem, which influences the cost of the investment and its recovery period, is the cooling mode of the condenser. Water cooling is more efficient from an equipment point of view, but requires greater expenditure for the arrangement of the water source, in addition to the expenses related to the operation of the pumps and the necessary environmental approvals. Air cooling requires larger heat transfer surfaces, but requires nothing more than the expenses related to the operation of the fans. However, air cooling is strongly dependent on the atmospheric temperature, ensuring a high efficiency of the installation only in cold periods. In the warm season, the efficiency of the installation can be maintained at an acceptable level by using coolers that can operate in wet mode (hybrid or adiabatic coolers).

In any case, the temperature of the discharged geothermal water is quite high, around 45…50°C, so there is still a high, unused thermal potential. On the other hand, the low thermal efficiency, 6…8%, highlights the fact that more than 90% of the heat taken from the geothermal water is discharged into the environment by cooling the condenser. It is interesting to study how to exploit this low thermal potential by joining this type of installation to a relaxation and spa treatment unit or by integrating the ORC installation into local heating system.

With the help of classic technology (Rankine cycle with steam) only high-enthalpy geothermal resources can be exploited: steam obtained directly from the geothermal reservoir (dry-steam), steam obtained by expanding hot geothermal water with high pressure (flash-steam) or steam obtained by injecting water into hot rocks located at great depth. The huge potential represented by medium and low enthalpy geothermal resources cannot be harnessed by conventional technology for the purpose of electricity production. The Organic Rankine Cycle (ORC) allows electricity to be obtained from these resources as well, being a smart means of producing electricity locally from renewable resources without greenhouse gas emissions. Benefiting from today’s chiller and heat pump technology, the cost of equipment investments can be quickly amortized. Moreover, expanders can be made relatively simply by adapting compressors used for refrigerants. The disadvantage of using ORC technology for the production of electricity from medium and low enthalpy geothermal water consists primarily in the very low efficiency, of the order of 4…7%, which makes a large part of the energy of geothermal water remain unused. Also, the electrical power obtained is small (from several tens of kW to several hundred kW), depending on the available flow rates of the geothermal water wells. Also, an important disadvantage is the high cost of drilling and setting up geothermal water wells, which can amount to several million EUR in the case of drilling at depths of 3000…4000 m. In the case of aquifers that cannot be exploited artesian, and which assume the reintroduction of waste water into the geothermal reservoir, the required pumping energy can reach the level of the energy produced by the installation, or even exceed it. Consequently, a careful analysis of local conditions is necessary to determine whether such an installation is economically justified, even if it is very tempting as an idea and as a challenge. Considering the fact that the geothermal water used still has an important thermal potential, coupling an ORC installation that produces electricity with a system for direct use of this potential (SPA units, greenhouses, fish and animal farms or as an energy source for heat pumps) can constitute an intelligent cogeneration system for urban communities in the vicinity of such resources.