Index

Appendix IV - Power Generation #

The majority of central electrical power generation is done by thermal plant, where heat energy is converted to electric power. The heat energy is generally used to raise the temperature of a working fluid (typically steam), and consequently its pressure. This high-pressure working fluid is then passed through a turbine coupled to an electrical generator. The energy available to rotate the electrical generator is a function of the turbine input and output pressures of the working fluid. Since the pressure of a gas is proportional to its temperature, the turbine input and output gas temperatures regulate the amount of power that can be generated. The Carnot efficiency for power plant is described by:

$\eta = 1 - \frac {q_C} {q_H} = 1 - \frac {T_C} {T_H}$

Where $${\eta}$$ is efficiency, $${T_C}$$ is the temperature at which the fluid is condensed and $${T_H}$$ is the temperature of the working fluid exiting the boiler. This relationship makes it clear that power plant efficiency can be increased by increasing the temperature of the boiler or lowering the temperature in the condenser. Lowering the condenser temperature makes it possible to use boilers that operate at lower temperatures.

The environment in which the power plant is situated typically determines the condenser temperature. This may be the ocean, a river or the ambient air, as moderated by evaporation of water in a cooling tower. One of the positive feedback effects of a warming environment is that thermal power plant becomes less efficient, typically when demand for more power in the form of air conditioning may increase. Since the environment has historically determined the condenser temperature, engineering solutions to increase power plant output has been limited to increasing plant size and increasing the boiler temperatures. This proposal offers a brand-new avenue – creating a stable, low temperature heat sink well below ambient temperature.

The technology outlined here can be coupled to any thermal power plant with a view to increasing its efficiency and thus increasing its rated output. Another option similar to Ocean Thermal Energy Conversion (OTEC $${[^8]}$$ ) is briefly described below.

OTEC utilizes the temperature difference between cold deep ocean temperature and the relatively warmer surface ocean temperature to boil and condense ammonia to run a turbine. This process requires input power to pump water onto land from both pools. The temperature difference is extremely small, giving very low plant efficiency, especially considering the large volume of water that has to be pumped from the ocean depth to drive the process.

It is proposed to consider low temperature (around 50°C) geo-thermal heat as a renewable heat source to drive the boiler, and utilizing the final output temperature of the CHE to condense the working fluid. The fluid proposed is ethanol since it has a lower heat of vaporization than both water and ammonia. This property can be utilized to boil ethanol at the surface and condense it at altitude. The condensate (liquid ethanol) can then be used to drive a turbine, with the liquid discharge being returned to the boiler to close the loop. This is depicted in Fig.5.

The final power output can be increased by raising the height of the boiler, with re-heating exchangers being distributed at different levels to prevent condensation as a result of expansionary cooling as the vapor travels up the stack to the condenser at the top.

The condenser output can be routed through heat exchangers placed above the HXT’s in the CUTs, pre-cooling the condensate to ground level ambient air temperature (25°C in the case study), before final cooling by the Stage 5 Output Stream at -6°C. In this way the waste/condenser heat from the power plant can assist in powering the CHE.

OTEC utilizes 28°C surface ocean temperatures to fuel the boiler and 5°C deep ocean water to condense the ammonia working fluid. The average geo-thermal temperature profile is about 25°C per km of depth below the ground level. Thus 50°C temperatures are available by “mining” low quality geo-thermal heat, which is technically readily achievable. The author spent a short period of time in the 1980’s working for AngloGold in South Africa, where several of their gold mines where at the 2km depth or deeper. The Mponeng mine currently operates around 3.5km depth and has plans to go down to 4.2km.

Geothermal heat is renewable, but is a resource that can be depleted if drawn from too heavily. The geothermal heat can be replenished with low quality solar thermal heat. The geothermal resource can thus be managed like a heat “battery”, with day time solar power used to replace heat that is extracted at above the natural geo-thermal heat replacement rate. A benefit from the CHE that serves this purpose is that the CHE expels a continuous flow of warm dry above the CHE locale, meaning that cloud local cover should be greatly reduced, if not eliminated. This means that solar thermal production could be relatively unaffected by cloud shading, resulting in predictable solar heat production. This should probably be studied with weather modeling to be verified.

To summarize, the power plant is proposed to run as follows:

• Boil/vaporize ethanol at below atmospheric pressure and 50°C into a tall vertical column
• Reheat the rising vapor at the necessary heights to prevent condensation formation as a result of expansionary cooling
• Condense the vapor to liquid ethanol at the top of the column utilizing the -6°C output of the CHE. The heat of condensation is used to partially power the CHE.
• Run the liquid ethanol through a series of “hydro” turbines coupled to electrical generators.
• Re-boil the ethanol in the closed loop system.
• The higher the vapor column, the greater the potential energy is of the condensate ethanol. Increased height will require more re-heat energy to counter expansionary cooling, so no thermodynamic laws are broken.

[8] Ocean Temperature Energy Conversion #

https://www.scientificamerican.com/article/hawaii-first-to-harness-deep-ocean-temperatures-for-power/