Topic 5: Other forms of Renewable Energies
Learning Objectives  Describe the use of geothermal energy  Describe the use of solar energy  Describe the use of fuel cells

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5.1 Geothermal Energy
Geothermal power (from the Greek roots geo, meaning earth, and thermos, meaning heat) is power extracted from heat stored in the earth.
Geothermal power is cost effective, reliable, sustainable, and environmentally friendly, but has historically been limited to areas near tectonic plate boundaries. The Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a very small fraction of it may be profitably exploited.

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5.2 Categorization of Geothermal Systems
Geothermal systems can be categorized as follows: 1. Hydrothermal or geohydrothermal Conditions in which underground water is heated and or evaporated by direct contact with hot porous rock. Hydrothermal systems that produce steam are labeled as vapor dominated while hydrothermal systems that produce hot water or a mixture of hot water and steam are called liquid dominated. 2. Geopressurized Associated with sediment-filled reservoirs that contain hot water confined under high pressure. 3. Magma Molten rock at accessible depths is contained in pools under active volcanoes. 4. Enhanced Geothermal Systems (Hot Dry Rock) Energy in hot dry rocks can be harvested by injecting and circulating water into the rock and use the steam to drive a turbine to generate electricity. To increase surface area, hydrofracturing the rock is used.
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5.3 Hydrothermal Systems
Dry steam plants use hydrothermal fluids that are primarily steam. The steam travels directly to a turbine, which drives a generator that produces electricity. The steam eliminates the need to burn fossil fuels to run the turbine (also eliminating the need to transport and store fuels). These plants emit only excess steam and very minor amounts of gases.
Dry Steam Vapor-dominated Hydrothermal System

The challenge is that the impurities in the steam can cause malfunction in the turbine and the gases emitted may be toxic. One solution is to install centrifugal separator to separate out the impurities and toxic gases
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5.3 Hydrothermal Systems

Flash Steam Liquid-dominated Hydrothermal System

Flash steam plants are the most common type of geothermal power generation plants in operation today. Fluid at temperatures greater than 182°C is pumped under high pressure into a tank at the surface held at a much lower pressure, causing some of the fluid to rapidly vaporize, or "flash." The vapor then drives a turbine, which drives a generator. If any liquid remains in the tank, it can be flashed again in a second tank to extract even more energy. The Flash Steam plants face the same challenge as that of the Dry Steam plants.
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5.3 Hydrothermal Systems
Binary cycle geothermal power generation plants differ from Dry Steam and Flash Steam systems in that the water or steam from the geothermal reservoir never comes in contact with the turbine/generator units. Low to moderately heated (below 200°C) geothermal fluid and a secondary (hence, "binary") fluid with a much lower boiling point than water pass through a heat exchanger. Heat from the geothermal fluid causes the secondary fluid to flash to vapor, which then drives the turbines and subsequently, the generators. Binary cycle power plants are closedloop systems and virtually nothing (except water vapor) is emitted to the atmosphere.
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Binary Cycle Liquid-dominated Hydrothermal System

Topic 5: Renewable Energy

5.4 Geothermal Heating
Geothermal heating is the direct use of geothermal power for heating applications, like space heating, for heated pools and industrial & agricultural applications. Thermal efficiency is high since no energy conversion is needed, but capacity factors tend to be low (around 20%) since the heat is mostly needed in the winter. A geothermal heat pump is used in geothermal heating.

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5.4 Geothermal Heating
A geothermal heat pump is a central heating or cooling system that pumps or transfers heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). Heat pumps can transfer heat from a cool space to a warm space, against the natural direction of flow, or they can enhance the natural flow of heat from a warm area to a cool one.

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5.5 Solar Energy
Solar energy can be harvested using either Photovoltaic cells or Solar Thermal Collectors. Solar Thermal Energy (STE) is a technology for harnessing solar energy for thermal energy (heat) wheareas Photovoltaics (PV) technology converts solar energy directly into electricity.

Solar PV

Solar Collectors

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5.5 Solar Energy
Solar thermal collectors can be categorized as low-, medium-, or hightemperature collectors. - Low temperature collectors are flat plates generally used to heat swimming pools or space heating. Collectors can use air or water as the medium to transfer the heat to their destination. - Medium-temperature collectors are also usually flat plates but are used for creating hot water for residential and commercial use. Solar drying, solar cooking and solar distillation are some applications. - High temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production.

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5.6 Fuel Cells
A fuel cell is an electrochemical cell that converts a source fuel into an electrical current and water. It generates electricity inside a cell through reactions between a fuel and an oxidant, triggered in the presence of an electrolyte.
The electrolyte is a substance specifically designed so ions can pass through it, but the electrons cannot. The electrolyte substance usually defines the type of fuel cell.

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5.6 Fuel Cells
Fuel cells are different from conventional electrochemical cell batteries in that they consume reactant from an external source, which must be replenished – a thermodynamically open system. By contrast, batteries store electrical energy chemically and hence represent a thermodynamically closed system. Many combinations of fuels and oxidants are possible. A hydrogen fuel cell uses hydrogen as its fuel and oxygen (usually from air) as its oxidant. Other fuels include hydrocarbons and alcohols, example direct methanol fuel cell. Other oxidants include chlorine and chlorine dioxide.

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Summary
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Describe the use of geothermal energy  Describe the use of solar energy  Describe the use of fuel cells

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