**2.2 Flash steam plants**

Currently, most of GPPs are flash steam plants. They use geothermal reservoirs with mixture sources, i.e., vapor and liquid-dominated (water) to generate an electricity. It means that the temperature and enthalpy of vapor and waters are lesser than the critical point. Therefore, the flash units are generally basic approach to change over the geothermal energy into power. Firstly, in this system the steam is separated from water using a cylindrical cyclonic pressure tank with a base loss of pressing factor. Then, the dry steam leaves the separator, flows to powerhouse and rotates the steam turbine. The vapor quality that defines the flashed fluid is given as:

$$\infty = \frac{m\_{vapor}}{m\_{liquid} + m\_{vapor}} \tag{1}$$

where: *mvapor* – mass of vapor and *mliquid* – mass of liquid.

### *Geothermal Power Generation DOI: http://dx.doi.org/10.5772/intechopen.97423*

The vapor quality value changes from 0 to 1 and commonly given as percentage. When vapor quality between 0 and 1, wet steam is then obtained. When the vapor quality is equal to 1, it is called "the saturated vapor" state.

There are two types of flash steam geothermal plants:


In a single-flash steam GPP the mixture fluid is flashed only in one separator. Schematic process diagram of the single-flash power plant is shown in **Figure 2**. The process of electricity generation in this power plant is accomplished as follows: hot water from production well is piped to flash separator (FS) by decreasing its pressure. In FS the steam is separated from hot water and transmitted to the steam turbine to spin it and convert mechanical energy to electrical by a generator. While the cooled steam in a turbine condenses to the water by the condenser while a part of the liquid from FS are reinjected to injection well.

Unlike single-flash steam, in double-flash steam the flash process of the fluid is applied in two separators. Although these plants are more expensive and more labor-consuming, however, they are preferable than single-flash plants as they generate 15–25% more electricity for the same states of fluid reservoirs [8, 9].

Schematic operational diagram of the double-flash power plant is illustrated in **Figure 3**. The fluid flows from well to a high pressure flash separator where from a mixture fluid the steam is separated and is piped to a two-stage turbine; another part – saline liquid is throttled down to the second separator. In the low pressure separator like the first one a partly boiled liquid again is separated to a steam and water. As a result, the steam gets directed to the low-pressure turbine. By keeping the pressure in the condenser, the steam from the low-pressure turbine is cooled using a sprayed cold water. Then, water is reinjected to injection well, as well as the cold water from the condenser.

**Figure 2.** *Schematic diagram of single-flash power plant.*

**Figure 3.** *Schematic diagram of double-flash power plant.*

### **2.3 Binary cycle power plants**

"Binary" cycle refers to as a secondary separate cycle. For the geothermal resource, binary indicates that the geothermal fluid (water/steam) never comes in a contact with the prime mover. Geothermal binary power systems are suitable for electricity production from low underground heat source [10]. The binary plant in Alaska, as an example, utilizes a geothermal resource of 57°C [11]; yet generally, binary system designs can exploit an inlet temperature range between 80 and 170°C [3]. The secondary fluid, known commonly as working fluid, in the binary geothermal system operates in a conventional Ranking cycle; and the binary cycle is known as an organic Ranking cycle (ORC) when the used working fluid is organic [12]. In binary ORC power plants the geothermal fluid passes through a heat exchanger to heat another working fluid of a low boiling point e.g., pentane, zeotropic mixtures, etc. which in turn vaporizes and drives a turbine [13]. Electrical production through a closed-loop binary unit is shown in **Figure 4**.

The standard working mechanism of a basic geothermal binary system can be summarized as: when the geothermal brine is pumped through the production well,

**Figure 4.** *Illustrative schema for a binary geothermal plant.*

#### *Geothermal Power Generation DOI: http://dx.doi.org/10.5772/intechopen.97423*

the heat extraction process is accomplished after passing through different components of the primary cycle. The geothermal fluid is initially filtered via sand removers to pass through the heat exchanger i.e., the evaporator/vaporizer and preheater, and finally pumped back into the reservoir by the injection well. On the other side of the secondary cycle, the pressurized working fluid turns into boiling state in the preheater. It then exits the vaporizer as a saturated vapor that subsequently expands in the turbine driving a power generator. The low-pressure working fluid vapor exiting the turbine is finally condensed in the ACC (air-cooled condenser) and pumped back to the vaporizer, closing the loop system and repeating the process continuously. Thus, the thermodynamic process of the low-boiling-point working fluid starts when it expands into the turbine in saturation vapor state, and completed when it is cooled through the condenser and pumped back (as a saturated liquid fluid) to the heat exchanger to emerge as a saturated vapor again [14, 15].

The efficiency of geothermal ORC, for high enthalpy field, can go as high as 23% [16]. However, cycle configuration plays a key role in thermodynamics of a binary power plant. Many performance and optimization studies have been recently carried out to examine the optimal configuration of ORC geothermal power facilities [17, 18] as well as on the investigation of optimal working fluids in ORCs [19–21]. In Ref. [22], the researchers investigated the performance of three configurations of ORC for binary geothermal power plants; simple ORC, regenerative ORC and ORC with Internal Heat Exchanger (ORC-IHE). It is concluded that the ORC-IHE outperforms the other configurations from the thermodynamic perspective while the simple ORC had the highest value of net output power. The 2-stage designs of a binary cycle yield higher net electrical power output and thermal and exergy efficiencies than the 1-stage counterparts [23].

As the thermal energy extracted from underground field is conveyed to a second working fluid; therefore, selection of such working medium plays an important role on the system design, performance, and economics. The optimal choice of the working fluid for a binary cycle must consider the thermodynamic characteristics of both geofluid and working fluid, safety of use, health and environmental impact [9]. Various objective functions have been used in literature for working fluid selection, such as the net power output [24], ratio of net power output to heat exchanger area [25], first or second law efficiencies [26] and volumetric expanders [27]. In [28] the authors conducted a comparative study of several working fluids, such as water, coolants and some hydrocarbons, for a Rankine cycle operating at low temperature. The study concluded that using organic working fluids, the Rankine cycle achieved good efficiencies for the recovery of low enthalpy resources. An optimization study conducted by [29] revealed that the n-pentane working fluid produced the highest first and second law efficiencies for a binary ORC plant. The study in [29] explored the thermodynamic performance of 20 working fluids for a binary ORC and found out that R123, R141b and ethanol are the most appropriate for small scale domestic Combined Heat and Power (CHP) applications. CHP or cogeneration plants are efficient technology that produces both electricity and thermal energy at considerably higher efficiency than its counterpart of only-electricity or only-heat systems.

It was reported that the operational parameters of a binary plant (such as air mass flow rate, mass flow rate of organic medium and inlet turbine pressure) and plant performance i.e., net power output degrade over the plant lifetime [14]. In order to maintain the plant performance over its life span, the mass flow rates of organic fluid and air cooling should be adjusted. In addition, the plant design can be modified by placing a recuperator and reducing the heat transfer area of vaporizer and preheater.

Besides the standard binary geothermal power system, advanced configurations of geothermal energy conversion systems have been also well investigated. This includes: hybrid single-flash and double-flash systems, hybrid flash-binary configuration and hybrid fossil geothermal technology [23, 30]. In addition, the development of hybrid power systems integrating geothermal plants with biomass, fuel cells, wind, solar systems and waste-to-energy (WTE) technologies has been gaining a lot of interest [31–35].

When compared to single-flash and double-flash cycles, the binary ORC plant attained the highest thermal efficiency and output power among the three geothermal power plants [36]. An exergoeconomic investigation between double-flash and single-flash/ORC combined cycles revealed that the single-flash/ORC integrated cycle offers the highest energy and exergy efficiencies [37]. Based on a comparison between various types of geothermal power plants in terms of energy and exergy metrics, it was concluded that the combined flash-binary cycle with R123 working fluid, at a temperature 230°C and mass flow rate of geothermal heat source 1 kg/s, has the highest amount between various investigated configurations with a maximum thermal efficiency of 11.81% [38].

A combined flash-binary ORC power unit is schematically shown in **Figure 5**. The working mechanism of the plant is as follows; firstly, the geofluid is throttled in a valve to a lower pressure (point 2), then the obtained two-phase fluid is decomposed into saturated liquid and saturated steam by getting through the flash chamber i.e., separator (3 and 4). The extracted saturated steam drives a prime mover that is connected to a power generator (5); the steam turbine exhaust is then cooled in the condenser (6). On the other hand, the saturated liquid of the separator enters the heat exchanger to give off heat to the binary unit (7). The exit mixed stream of condenser and heat exchanger is then injected back into the ground (8). The pump

**Figure 5.** *Schematic diagram of the single flash/ORC combined cycle.*

#### *Geothermal Power Generation DOI: http://dx.doi.org/10.5772/intechopen.97423*

within the binary ORC pressurizes the organic working fluid to a high level (10), that is in turn be in a form of saturated vapor when thermally exchanged with the heat of saturated geofluid stream in the evaporator (11). The saturated vapor is expanded into the steam turbine and delivers work to produce further electricity (12). Finally, water flow in the condenser condenses the superheat vapor and exits as a saturated liquid (9) [39].

As mentioned earlier, integration of multiple generators of different technologies especially of renewable ones has been attracting a considerable attention. The synergy offers cost competitiveness, greater overall efficiency and a higher capacity factor as compared to a single source power supply [40]. For instance, hybridization of geothermal with concentrating solar power (CSP) can overcome several challenges encountered by standalone geothermal plants [41]. The concept is that as the ambient temperature increases with the progress of the day the hourly output of a standalone geothermal plant decreases. Nevertheless, CSP involvement can handle this issue as its output increases with a rise in the ambient temperature and more than 70% in annual energy output could be attained [42]. Generally, CSP can be incorporated in the geothermal plant both in the preheating or the superheating configuration [43].
