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Geothermal Engineering Research Office

Low grade heat

The low efficiency inherent in low temperature electricity generation is offset by huge amounts of energy available for direct heat use, such as:

water heating; space heating; space cooling; pool heating; industrial drying; food processing; fish farming

The energy demand for heat < 100 °C is usually the largest part of the total energy use profile.

GEROM - Pool Heating GEROM - Food Processing

Estimated Domestic Energy Profile[5]


‘Extra energy’ for direct use

In example (a) above, if we consider the heat available at 40 °C to 120 °C, we have z2 = 5830 m, z1 = 1380 m and z2 - z1 = 4450 m (as before).

Thus, we have an extra useful heat content Eo = 3610 PJ ˜ 86 Mtoe ˜ 1003 TWhth available for direct use.

If for simplicity we take the extra water flow to be also 10 m3 s-1, the time constant t remains 60.5 years, and the useful heat extraction rate (initially and after 25 years) is again another 1.89 GWth and 1.25 GWth respectively.

GEROM - Baby Bath GEROM - Shower

GEROM - GeyserOverall efficiency

Therefore when considering efficiency, the total efficiency up to the end user must be considered.

Fossil fuel power plants are more efficient electric power generators than low temperature plants.

Squandering the bulk of that electricity for water heating, space heating, and air conditioning drastically lowers the overall efficiency.

Thermal vs. electrical applications

GEROM - Shower

It is irrational to generate grid quality electricity from fuels, wasting the majority of the energy as thermal emission, incur further losses in cable distribution, and finally dissipate the electricity as heat, when the dominant end use is warm water and ambient heat/cold!

The same comments apply to Wind and PV electrical generation, which should cater purely for electrical power and lighting applications and not for heating.

Ambient heating and water-heating are more efficient from direct heat production with local distribution. Combined heat and power generation is even better. 

Example of Direct Heat Use[1]

GEROM - Direct Heat Usage

Utilization of geothermal fluids[1]

GEROM - Temperature and Processes

Cascading energy use[1]

GEROM - Cascading use of Geo heat


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