Geothermal and Heat Pumps
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Geothermal electricity
The core of the earth is very hot, the inner core is between 4400 to 6000 Celsius (measurements are understandably a little difficult to make). The outer core is made of liquid iron and nickel and has slightly lower temperature 4500-5500 Celsius.
This heats up also earth’s mantle and in some places the mantles molten rock can travel all the way to the surface resulting in volcanoes.
By Kelvinsong - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23966175
There is practically limitless source or energy below our feet. Today geothermal electricity is produced in 24 countries world-wide in places where it is most cheaply available (US, Indonesia, Philippines, Island in Europe etc.). Quite little has been invested to harness it elsewhere. It would present a stable source of energy unlike intermittent ones that are politically preferred today.
The challenge today is that building geothermal energy stations can be expensive.
There are a few different types of geothermal power stations.
Dry steam systems use geothermal reservoirs that produce hot steam to be used directly for electricity generation. The steam collected is hot <235 Celsius and the heated water vapour is funnelled directly into a turbine that drives an electrical generator. They are called dry because there is no water in the steam.
Other plants (flash stream and binary cycle designs) use a mixture of steam and water.
Single and double flash system us cooler steam <180C and water present mixed with steam. In double flask system hot water from first turbine can be used as input for 2nd stage.
In binary cycle plant heat exchanger is used to primary system and steam is generated in second circuit. Advantage of this design is that a lower quality heat source can be used
Geothermal energy is environmentally friendly, renewable and stable. There is rapid evolution albeit much less investment that to other renewable technologies and not much in the main stream media.
On the negative side it is location dependant at least with current technology readiness and those areas tend to have risks of earthquakes.
Heat pumps
Heat pumps also move energy from the environment to be used, but in much smaller scale and are used for heating houses.
Heat pumps extract energy from cold source to a hot source. This sounds counterintuitive but they work because very cold fluid flows in pipes and able to extract heat from the cold side. The cold side is typically at -40 Celsius.
It is partially renewable energy. General rule of thumb is that 1 kWh of electricity can produce about 3 kWh of heat.
There are different kinds depending where the heat is collected from. Air to heat pump use air as the name says. They extract heat from outdoor with evaporator and collect it with indoor heat exchanger coil (condenser). When temperature goes down, so does the efficiency of air to heat pumpups. Coldest days require additional heating.
Ground source heat pump collect energy from the ground around houses. Pipes are either vertical or horizontal. Vertical wells can be up to 100-200 m but often much less. Horizontal are cheaper to install, but vertical take less space and are more efficient. Ethanol is used as circulating fluid.
Exhaust air pump. Here heat is pumped from indoor air. They require minimum air exchange 0,5 l/h. The house always needs additional heat source, they just lower your energy costs.