Review of technologies for hybrid uses of
geothermal and additional RES
Activity 3.2 in WP 3
2
Index
INDEX 2
SUMMARY 3
1. GEOTHERMAL SOURCE AND SOLAR ENERGY 4
1.1. Solar-geothermal hybrid plant based on an ORC 4
1.2. Micro CHP systems operating with geothermal and solar energy 6
2. GEOTHERMAL SOURCE AND BIOMASS 9
2.1. Technologies 9
3. GEOTHERMAL SOURCE AND NATURAL GAS AND BIOGAS 13
3.1. Technologies 13
4. GEOTHERMAL SOURCE AND WASTE HEAT 15
4.1. Technologies 15
REFERENCES 18
3
Summary
Low-enthalpy geothermal resource, as the name suggests, is a source of relatively low-temperature geothermal water. According to various authors as well as a variety of the limit to which the temperature geothermal resource is low-enthalpy resource. We will assume that the temperature is up to 150°C.
Further in this report is presented the technology that could be used for the electricity generation from low-enthalpy geothermal resource with an additional renewable energy source such as solar, biomass, biogas and waste heat. The second energy source is mainly used for medium or steam additional heating. These renewable energy sources are chosen because of most efficient use in the area of South-eastern Europe.
4
1.
Geothermal source and solar energy
In the beginning there is presented solar-geothermal hybrid plant based on an Organic Rankine Cycle (ORC). In the second part, two micro CHP systems operating with geothermal and solar energy are described.
1.1. Solar-geothermal hybrid plant based on an ORC
This solar-geothermal integration for electricity generation was proposed by Lentz and Almanza (2006a,b) for possible application in Mexico. The plant which was proposed is based on a single or double flash cycle, where steam turbine expands the steam produced in flash separators from the geothermal brine. There were suggested different configurations to enhance steam production with solar energy (but all based on the Direct Steam Generation – DSG), where the geothermal brine is evaporated by flowing directly in the solar collectors. The problem is aggressive species in most geothermal brines, such as CO2, H2S, SO2, HCl or NH3 [1].
Reference cycle definition and on-design performance
Binary units (based on ORCs) are becoming increasingly common. In Fig. 1 the supercritical regenerative ORC is shown. As working fluid was chosen R134a because it is a widely used, nontoxic and nonflammable fluid. Assumed temperature from the geothermal brine is 150°C, waste brine is injected at not less than 70°C to minimize cooling of the geothermal reservoir. The efficiency of the ideal process, i.e. the efficiency of the Lorenz cycle receiving heat between 150°C and 70°C and discharging heat at ambient temperature (Di Pippo, 2004), is equal to 24,5 %. The second-law efficiency of the cycle at on-design conditions is 55,5 % [1].
Fig. 1: Schematic of the assumed supercritical regenerative ORC and its representation on the temperature-entropy plane. [1]
5 Heat exchanger profiles are displayed on a temperature-heat plane in Fig. 2.
Fig. 2: Temperature-heat diagram of the ORC plant heat exchangers. Heat is expressed as the percentage of the heat entering the cycle from geothermal brine. [1]
Off-design performance
An additional heat exchanger placed after the main heat exchanger of the base plant was taken into consideration. This additional heat exchanger allows the mass flow rate and/or the maximum temperature of the R134a to increase. The off-design analysis is a general thermodynamic assessment of the effects of an additional heat source (derived from for example solar field) on the ORC cycle. In Fig. 2 schematic of the hybrid configuration for off-design calculations is shown [1].
Fig. 3: Schematic of the hybrid configuration used for off-design calculations and its representation on the temperature-entropy plane. [1]
6 Plant should be operated to achieve the highest conversion efficiency of the additional heat available. Optimal combination of max temperature and mass flow rate can be identified for any value of the additional heat. The temperature-heat diagram of the plant heat exchangers for two specific off-design conditions with different maximum cycle temperatures is shown in Fig. 4.
Fig. 4: Temperature-heat diagram of the plant heat exchangers for two specific off-design conditions with different maximum cycle temperatures. Heat is expressed as the percentage of the heat entering the cycle
from both geothermal and solar source. [1]
1.2. Micro CHP systems operating with geothermal and solar energy
The performances of the geothermal ORC plants were measured in terms of first and second law efficiencies. Energy efficiency is usually in the range 5 – 15 % while exergy efficiency is typically in the range 20 – 54 %. Below are described two innovative CHP ORC systems powered by low-temperature geothermal resource (about 90°C) and solar energy captured by solar collectors. The reference system is designed to produce 50 kWe. The electrical efficiency of the cycle is expected to be low (around 9 %) [2].
Single-pressure configuration with heat transfer fluid
Organic fluid coming from the condenser is preheated in a heat exchanger (geothermal heater) by a geothermal flow rate at 363 K. Then the organic fluid is heated up to the 420 K passing through a heat exchanger connected to an Evacuated Tube Collectors (ETCs) field. After producing mechanical work in the turbine, the organic fluid has still high temperature and enthalpy values. The organic fluid is then fist cooled from superheated conditions down to saturated vapour
7 conditions at variable temperature (DSH). The DSH can be connected to a domestic heating system. Organic fluid is then circulated to the geothermal heater to start the cycle again. Described process is shown in Fig. 5 [2].
Fig. 5: Single-pressure geothermal/solar ORC layout. [2]
The thermodynamic cycle for R134a as a working fluid is shown in the T-s diagram in the Fig. 6.
Fig. 6: Thermodynamic cycle for R134a- [2]
Dual-pressure system with direct-steam collectors, using two different solar collectors
The organic fluid is directly heated by the ETC and then by the Parabolic Trough Collectors (PTCs) fields. The ETCs are used only for liquid preheating and phase transition with a recirculation loop producing vapour. A vessel is used for separating the saturated vapour phase, part of which is directed to the low-pressure turbine (LPT). An appropriate amount of the saturated liquid phase in the separation vessel is compressed and directed to the high-pressure loop using PTCs. After LPT
8 the organic fluid is cooled in De-Super Heater (DSH) and condensed to start the cycle again. The system is shown in Fig. 7 [2].
Fig. 7: Dual-pressure geothermal/solar ORC layout. [2]
In all solar power systems, the solar filed represents the main cost item, accounting for about 80 % of the electricity cost, which leaves the solar source still uncompetitive with fossil fuels for power generation, however integration with a solar field could improve the attractiveness of the many low-enthalpy geothermal sources [1].
9
2.
Geothermal source and biomass
2.1. TechnologiesHybrid technologies for coupling geothermal and biomass sources are in early development stages. The development of hybrid biomass-geothermal systems therefore, the use of combined technologies, is a research priority in Europe [3]. To date, little information is available on implemented plants and on the theoretical aspects. Presented herein is the available information for operating plants followed by the description of plants that have been proposed in the literature.
The first ever installed hybrid geothermal-biomass plant is located in Wendel, California (USA) and is still operating. In this plant, called Honey Lake Power, geothermal heat is used for drying wood chips and for preheating fluid for a 35.5 MW biomass unit [4,5]. This steam cycle power plant uses 22 kg/s geothermal fluid at 118°C [6] and it contributes to about 1.5 MWe of the total plant output [7]. The final report [8] of the project on the hybrid plant gives additional information. Additional electrical energy is produced by a binary cycle due to isobutene heating. This heating process is first provided by a geothermal fluid heat exchanger and then by a heat exchanger receiving extraction steam from the biomass boiler. The working fluid feeds the turbine (hydrocarbon expander) that is connected to a generator that finally produces energy. The vaporized working fluid is transferred to a condenser for cooling and successively returned to the geothermal heat exchange. In Austria in the area of Neukirchen an der Vöcklaa combined use of geothermal and biomass energy sources has been operating since 2012. The district heating of the town comes from the new geothermal scheme and a biomass plant. The geothermal scheme consists in a deep well (downhole temperature of 105°C). The energy from the hot treated water is extracted through a heat exchanger and the heat is then sent along a 1 km pipeline which operates a biomass heating plant and a district heating system. The biomass plant uses local timber [9]. Another combined system for district heating will be operating in Hungary. The combined system will be composed by a biomass boiler fed with wood chips (with plant capacity of 1.2 MW) and a geothermal well (with a water temperature of 60-65°C) combined with a heat exchanger [10]. Over the last years several different plants coupling geothermal and biomass energy sources have been suggested and hereafter a brief description of the proposed technology is presented. Most of the studies have proposed the utilization of Organic Rankine Cycle (ORC) technology.
Kohl and Speck [11] presented a system in which energy transfer is achieved utilizing three heat exchangers: a pre-heater, a steam generator and a steam super-heater (Fig. 8). For the two former heat exchangers the energy source is geothermal whereas for the latter biomass is the most likely source to be used.
10
Fig. 8: Configuration of the proposed hybrid geothermal-biomass plant (taken from [11]).
The authors tested the proposed hybrid system using ‘water vapor’, hexane and isobutene as working fluids (ORC) for the production of electrical energy.
Also Bombarda et al., [12](2010) proposed a hybrid geothermal-biomass plant based on ORC technology (see Fig. 9). They considered a geothermal source with temperatures ranging from 110 to 150°C.
Fig. 9: Configuration of the proposed hybrid geothermal-biomass plant (taken from [12]).
To transfer heat between the biomass-fired furnace and the power unit synthetic oil was selected (the assigned loop temperature is the temperature common for biomass plants). Furthermore, to test the proposed hybrid plant, data for the biomass source was taken from a plant located in the Molasse basin (Germany) that generates electrical and thermal energy through the year and according to seasonal demand, respectively. A cascade cycle was selected with octamethyl-trisiloxane (MDM), regenerative, and R245fa as working fluids for the upper and lower cycle, respectively. The results highlighted a higher net power production compared to separate plants. Borsukiewicz-Gozdur [13] assessed two hypothetical hybrid geothermal-biomass systems. For both systems the assumed geothermal water had low temperatures (80-120°C) with a constant flow rate (30 kg/s). The first plant consists in a dual-fluid- hybrid power plant (DFH) formed by an upper and lower cycle (Fig. 10).
11
Fig. 10: Configuration of the dual-fluid hybrid geothermal-biomass plant (taken from [13]).
In the upper cycle (Hirn cycle) the working fluid (water) is pre-heated and vaporized in a biomass boiler (u4 to u1), and subsequently directed to a steam turbine (u1 to u2) where the fluid expands and performs work. Then the fluid is condensed in a condenser-vaporizer exchanger (u2 to u3) and subsequently is moved to the biomass boiler by the circulating pump that pressurizes the fluid to the apt value (u3 to u4). In the lower cycle (ORC), connected to the upper one by the condenser-vaporizer, after preheating using geothermal sources, the working fluid evaporates in the condenser-vaporizer (b5 to b1) and then is directed to the vapor turbine (b1 to b2) that is linked to an electrical energy generator. Subsequently the working fluid is directed to the condenser (b2 to b3), circulation group (b3 to b4) and, finally, to the organic liquid heater (b4 to b5). The working fluids that were tested are: R365 mfc-water, R245fa-water and R236fa-water.
The second hybrid plant consists of a single-fluid hybrid power plant (HYB) (Fig. 11).
Fig. 11: Configuration of the single-fluid hybrid geothermal-biomass plant (taken from [13]).
This hybrid plant is fed by combustion of biomass in the upper part and a geothermal source is used to preheat the working fluid. For this proposed hybrid cyclohexane and water were tested as working fluid. The author highlights the advantage of the combined use of geothermal and biomass against only geothermal sources and that the best option, in the case the general purpose
12 is to use the least share of biomass derived energy, will be using a dual-fluid-hybrid power plant with R236fa as working fluid.
A technical and economic analysis of a proposed hybrid geothermal-biomass system for district heating was carried out for Cornell University (Ithaca, NY, USA) [14]. In Fig. 12 the schematic of the coupled energy system is shown. Low-temperature geothermal resource is considered.
Fig. 12: Configuration of the hybrid geothermal-biomass system. The ORC unit is not included in the 'DH' scenario. Lines are in red for hot supply water and in blue for cold return water (taken from [14]). The researchers evaluated two design scenarios: (i) the geothermal source was applied only for district heating (‘DH’) and (ii) the geothermal source was used for the combined generation of heat (for district heating) and electrical energy through an Organic Rankine Cycle (ORC) plant (‘DH-ORC’). The EGS reservoir and the district heating (DH) network are hydronically divided by a heat exchanger. Furthermore, a backup steam-to-water heat exchanger was inserted. In both scenarios, a boiler fed with torrefied biomass was used to produce the additional heat for the DH when needed. Heating and domestic hot water needs are fulfilled by the DH water supply pumped to building substations, from which, roughly the 30% of return DH water is cascaded to greenhouse substations. The cycle is closed by pumping the return water to the central heat exchanger.
In the proposed DH pressurized liquid water at temperatures ranging from 90°C to 140°C is used. The temperature of the DH supply can be lowered as in this case study it was constrained by the existing space heating system. Substations of the proposed system use plate and frame water-to-water heat exchangers. The hybrid system also has water-to-water circulation pumps and roughly 20% of the pipeline network is formed by pre-insulated pipes. In the greenhouses a soil heating system with a fluid temperature ranging from 30 to 60°C was considered. The greenhouses network consisted in polyethylene tubes and a finned coil air heater. Furthermore, the ORC unit consisted in two low capacity ORC power plants operating only during low heat demand conditions. For the ORC unit, the researchers considered 25 different fluids and selected R32 fluid.
In conclusion it must be emphasized that research is ongoing. For example, Enel Green Power is focusing in particular on geothermal in combination with biomass and solar energy and the “European technology platform: Renewable heating and cooling” has a Focus group specifically for renewable energy hybrid systems.
13
3.
Geothermal source and natural gas and biogas
3.1. TechnologiesThe geothermal energy and natural gas electricity production is one of the best combinations regarding CO2 emissions (global warming effects). The electricity production with combination of
geothermal and gas energy is very useful also regarding gas emission (Fig. 13). This term is especially important when we wish to produce electricity near big cities. Unfortunately, in scientific literature is not possibly to find many projects or plans, how to build geothermal and natural gas power stations in combination.
Fig. 13: The CO2 emissions regarding electricity production [15].
In principle it is possible to use processes as it as described in combination of geothermal with biomass, and solar energy, in this case is the added energy gas or biogas energy (source 2 in Fig. 14).
Fig. 14: Principle of application of natural gas with geothermal energy.
It is possible to combine natural gas-biogas and geothermal energy in many ways. One suggested way shown by MAN [16]. MAN combined gas engines with ORC process. In this way we could combine heat from exhaust gases with geothermal energy in ORC process.
14
Fig. 15: Two-stroke, low-speed diesel engine of MAN B&W design in combined cycle [16].
Cornell University [16] owns a state of the art natural gas combined cycle CHP plant, which has been fully operational since 2009. The objective in this study was to show an efficient heat production and distribution system which uses only geothermal, gas and biomass resources (Fig. 16) to cover demand of a fraction of the Cornell campus.
15
4.
Geothermal source and waste heat
4.1. TechnologiesWaste heat is energy discharged in the form of heat from the system (machine, facility, bio-system etc.) undergoing some kind of energy transformation. Waste heat is not targeted or desirable energy output, but sometimes it is necessary byproduct, such as in power plants operating by Rankine cycle (heat of condensation, or latent internal energy of exhaust flue gasses). There are a variety of waste heat sources with very different properties (geometry, heat capacity, temperature etc.), and those properties are determining regarding the possibilities for waste heat recovery. Nowadays the most actively researched option for waste heat recovery is the conversion of low-grade waste heat (heat sources with low specific exergy) to electricity utilizing Organic Rankine cycle [17-20], Kalina cycle [21,22] or some other methods [23]. Low-grade waste heat has the similar temperature range to low-temperature geothermal source, hence the way to make electricity out of those heat sources is quite similar as well [24-26].
Theoretically, if waste heat source is available, surplus waste heat can be used in different low-temperature geothermal energy systems, which include use in spas, heating and cooling systems, aquaculture, agriculture, snow melting, low wattage power generation, in order to improve their efficiency and/or power output. Specific solution is dependent on the properties of low-temperature geothermal source, waste heat source, the purpose of the system and cost effectiveness analysis.
Other possible way of using waste heat in low-geothermal applications is for “recharging” of the geothermal source.
For example, all systems for indirect use of geothermal energy can operate with additional heat source. If available, waste heat can be regarded as additional heat source. Depending on the properties of the low-temperature geothermal source and waste heat source, there are many different possibilities. Fig. 18 shows typical and simplified schematics of the heating system with indirect use of geothermal source and additional heat source (waste heat). These systems can be effectively used in agriculture or for greenhouses [27].
16
Fig. 17: Heating system with indirect use of geothermal source and additional heat source
In low-temperature geothermal binary plants utilizing ORC (Fig. 19), Kalina cycle, etc. waste heat from some other process can be used in different ways, such as preheating or superheating, in order to improve system efficiency or power output. If on the other hand ORC plant is primarily using low-grade waste heat, surplus low-temperature geothermal source can be used in order to provide steady heat supply to the system (as waste heat may not be always available or in the same quantity), hence enabling easier optimization of the system and increase of its efficiency. In those kind of applications one should be aware of low specific exergy of low-temperature ground source and low-temperature waste heat.
17 Other group of applications, which is fairly common, refers to “recharging” of low-temperature geothermal sources with waste heat. Basic principle can be explained on systems using ground source (low-temperature geothermal) heat pump for heating and possibly cooling, which are commonly called geoexchange systems. These systems account for around half of all geothermal use [28]. In the heating period soil is used as a heat source (low-temperature geothermal), and in the cooling period, it can be used as a heat sink1 (Fig. 20). In that way temperature degradation rate of low-temperature geothermal source during its lifecycle is reduced, which has positive effect on heat pump COP and system cost effectiveness in whole.
Fig. 19: a) Geoexchange system in the heating mode; b) Geoexchange system in the cooling mode
There are some other solutions similar to this concept, like hybrid energy system that includes a fuel cell and geothermal heat pump, that improves the efficiency of a ground-source heat pump by using low-grade heat from the fuel cell [29]. The system takes the waste heat from the fuel cell and incorporates the waste heat into the geothermal heat exchange system. The fuel cell provides electricity to the geothermal system and the digital control unit, allowing the system to operate off the grid. The fuel cell can also provide electrical power to the building. The temperature of the fuel cell is regulated by a digital control unit. Fuel cells are more efficient than generators. However, fuel cells typically waste about 50% of the input energy in the form of heat. In the heating mode, the waste heat from the fuel cell is pumped into the building for heating purposes. In the cooling mode, the waste heat is pumped into the ground where it is absorbed by the earth. This technology can improve the efficiency of the heating and cooling systems in the building and also has applications in remote or backup power generation systems for homes and commercial buildings.
1
Cooling can be passive or by the means of a heat pump. Either way, the heat removed from the cooled space is stored in the ground, thus recovering portion of the energy used from the low-temperature geothermal source during
18
References
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List of Figures
Fig. 1: Schematic of the assumed supercritical regenerative ORC and its representation on the
temperature-entropy plane. [1] 4
Fig. 2: Temperature-heat diagram of the ORC plant heat exchangers. Heat is expressed as the percentage of the heat entering the cycle from geothermal brine. [1] 5 Fig. 3: Schematic of the hybrid configuration used for off-design calculations and its representation
on the temperature-entropy plane. [1] 5
Fig. 4: Temperature-heat diagram of the plant heat exchangers for two specific off-design
conditions with different maximum cycle temperatures. Heat is expressed as the percentage of the heat entering the cycle from both geothermal and solar source. [1] 6 Fig. 5: Single-pressure geothermal/solar ORC layout. [2] 7
Fig. 6: Thermodynamic cycle for R134a- [2] 7
Fig. 7: Dual-pressure geothermal/solar ORC layout. [2] 8 Fig. 8: Configuration of the proposed hybrid geothermal-biomass plant (taken from [11]). 10 Fig. 9: Configuration of the proposed hybrid geothermal-biomass plant (taken from [12]). 10 Fig. 10: Configuration of the dual-fluid hybrid geothermal-biomass plant (taken from [13]). 11 Fig. 11: Configuration of the single-fluid hybrid geothermal-biomass plant (taken from [13]). 11 Fig. 12: Configuration of the hybrid geothermal-biomass system. The ORC unit is not included in
the 'DH' scenario. Lines are in red for hot supply water and in blue for cold return water
(taken from [14]). 12
Fig. 13: The CO2 emissions regarding electricity production [15]. 13
Fig. 14: Principle of application of natural gas with geothermal energy. 13 Fig. 15: Two-stroke, low-speed diesel engine of MAN B&W design in combined cycle [16]. 14
Fig. 16: Actual system of cogeneration [16]. 14
Fig. 18: Heating system with indirect use of geothermal source and additional heat source 16
Fig. 19: Organic Rankine cycle. 16
Fig. 20: a) Geoexchange system in the heating mode; b) Geoexchange system in the cooling mode 17
