Combining renewable energy technologies using their specific characteristics and assets can lead to a more efficient and increased power generation.
The renewable energy resources biomass, geothermal, hydro, solar, wind, etc. depend on certain weather, geographic, geological and other conditions – combining them by using synergy effects and strengths of certain technologies can thus lead to more successful projects. Major advantages of geothermal energy in particular are the factors reliability, compact site development and 24/7 baseload power. In the following three hybrid demonstration projects are shown:
- Ahuachapán power plant, El Salvador: A geothermal field and concentrated solar energy with a prototype array of parabolic mirrors are used for power generation.
- Honey Lake hybrid power plant, USA: Wood chips are the main fuel source of the 35.5 MW plant. Geothermal resources of 116°C are used to dry the wood chips and increase the temperature of the boiler condensate water.
- Stillwater hybrid power plant, USA: For peak addition to the 47 MW geothermal plant a 24 MW solar installation was added.
Ahuachapán power plant, El Salvador
The geothermal energy production in El Salvador dates back to 1975, with the first 30 MW Unit in Ahuachapán. In El Salvador geothermal has been one of the main sources of electricity in the country, supplying up to 41% of the national electricity demand in 1981 for example. Ahuachapán was the first geothermal field in El Salvador to be developed for commercial electricity generation.
Besides two 30 MW single-flash units a third double-flash unit came online in 1981, bringing the total capacity to 95 MW. Ahuachapán was the first geothermal field in El Salvador to be developed for commercial electricity generation (Guidos and Burgos 2012).
In 2007 LaGeo, the national electricity provider initiated a thermosolar R&D project at its geothermal plant Ahuachapán aiming at enhancing the output power of the geothermal facility.
Technological details at Ahuachapán
The thermosolar-geothermal hybrid system with its prototype array of 160 m2 of parabolic mirrors has produced 0.1 kg/s of steam with 99.8% quality at 4.4 bar-g and 154°C well head conditions. Two solar collectors concentrate the solar irradiation into an absorber pipe (see Figure 1).
The absorber pipe is filled with the Hot Temperature Fluid (HTF) Therminol 55, a thermal oil which can reach up to 290°C. It is circulated in the system at 1.5kg/s mass flow until it reaches 225°C, the minimum working temperature of the steam generator.
The research conducted by LaGeo concluded that the appropriate scheme is to install the steam generator in the separated water line between the cyclonic separator and the flasher system. A fraction of the geothermal liquid boils in the steam generator and the generated steam can be sent to the medium pressure steam line. The residual water which does not boil is sent to the current flasher tanks.
Cyclonic separation is a method used to remove particulates from the stream by using rotational effects and gravity in order to separate mixtures of solids and fluids. In the flasher the liquid is flashed to steam which drives the turbine generator.
A study of Alvarenga et al. (2008) concludes that the hybrid solar-geothermal plant could provide peak power during high-demand periods, but could not operate as a stand-alone power plant. The authors also discovered that the temperature of the HTF (243°C) is considerably higher than that of the geothermal fluid (225°C). Due to the lower efficiency of the local solar technologies, it is recommended to import high-tech solar equipment and heat exchangers.
The study of Handal et al. (2007) concludes that a combination of Concentrated Solar Power (CSP) and geothermal power facilities could present costs lower than those for hybrid solar-fossil plants. The high solar irradiation in El Salvador with almost 12 hours and global solar energy over 4.7 KWh/m2/day is very favorable for this type of hybrid system.
Honey Lake hybrid power plant, USA
The Honey Lake power plant is located in northeastern California in the town of Wendel. The plant was built in 1989 with Greenleaf Power taking ownership in 2010. The 30 MW plant has a long-term agreement with Pacific Gas & Electric for the power produced. The plant also utilizes the geothermal resources, making the plant a very efficient hybrid biomass-geothermal energy facility.
Wood chip waste such as forest thinning, logging residue, mill waste and other waste wood is used as main fuel source. The wood generally arrives with 50% moisture content. Geothermal fluid (1,600 m depth, 118°C, 22 kg/s) is used to preheat the boiler condensate water and to dry the wood chips.
Hereby, the fuel consumption is reduced and the boiler efficiency is increased. Approximately 19% of fuel is saved. Three wells are used: WEN-1, WEN-2, and WEN-3. The Honey Lake plant employs 25 staff and is indirectly responsible for an additional 85 jobs in the region.
Electricity is generated in two cycles, a wood-geothermal cycle and a binary cycle. The overall plant has the following components:
- Power generation cycles
- Wood-burning power cycle
- Geothermal power cycle
- Fuel supply and delivery processes
- Water supply, treatment, cooling, and discharge
- Fire protection systems
- Power transmission lines
Project components are shown in Figure 4, the location of wells, transmission lines, etc. are shown in Figure 5.
The wood-burning power cycle
The wood-burning power cycle employs a conventional wood-fired boiler to generate high-pressure steam. The steam boiler can provide 300,000 pounds per hour of superheated steam (950 degrees F, 1,280 pounds per square inch). This steam drives a turbine generator capable of producing approximately 34.5 MW of electricity.
The geothermal production cycle
In the binary cycle geothermal fluid heats the working fluid isopropane which vaporizes at a lower temperature. Extraction steam from the wood-burning cycle turbine is condensed in a heat exchanger and used to further heat the working fluid. Once fully heated the vaporized working fluid is piped to an expander (the turbine) where it expands and drives a turbine generator.
It has the capacity to produce 4 MWel. As in other binary cycles the vaporized working fluid is then transferred to a condenser, cooled by water from the cooling tower, and returned to the geothermal heat exchangers as a liquid to repeat the process.
The total capital cost of the project was initially estimated to be USD 71,160,000 including capitalized interest during construction, fees, interconnection costs, development expenses and taxes (GeoProduct Corporation 1988).
Stillwater Geothermal-Solar PV hybrid power plant, USA
The combined solar photovoltaic and geothermal power plant was built by Enel Green Power North America in the town of Fallon in Churchill County, Nevada. 89,000 polycrystalline silicon solar panels complement to the 47 MW geothermal plant at peak periods (Singh 2012) adding 26 MW to the plant’s output.
Enel expects that around 40 million kWh of clean energy are generated per year, enough to meet the needs of 15,000 American households as well as avoid the emission of more than 28,000 metric tons of CO2 into the atmosphere each year (Enel Green Power 2012). This is one of the first hybrid renewable energy projects which combine the continuous generation capacity of binary-cycle, medium-enthalpy geothermal power with the peak capacity of solar power.
The logic behind combining solar and geothermal power makes sense when one thinks about the strengths and weaknesses of both technologies. In the last few years, solar power has grown, thanks to a quick development turnaround, relatively low upfront risk for investors and a sharp drop in the price of solar panels.
High upfront geothermal development costs, the associated risks, but long-term success when finding a viable underground resource, however, makes the combination very resourceful.
Singh (2012) reports that Nevada’s renewable portfolio standard target of 25% by 2025 has gone a long way in expanding the state’s green sector in recent years. Northern Nevada, the center of the state’s geothermal activity, already generates about 24% of its energy from renewable sources – far above the national average of 13%. The Nevada legislature has also enacted several measures for fast-tracking renewable energy development in the state.
Hybrid renewable energy projects in combination with geothermal energy exist around the world and show that they are feasible and provide viable solutions. A combination of Concentrated Solar Power (CSP) and geothermal power facilities can present lower costs than those for hybrid solar-fossil plants.
Further commitment is required by industry and research to investigate in projects which focus on non-conventional energy sources. In countries with high solar radiation and favorable geothermal resources such as El Salvador it is recommended to combine these two renewable energy resources.