Tour Bi-Directionnelle - Rohatensky
The Solar Heat Pump Electrical Generation System (SHPEGS)
Page en construction et incomplete, disponible en anglais seulement pour le moment
de Robert J. Rohatensky
(fragment of the original page)
see also on PESwiki : Energy Tower Perpetual
and Bi-directional Energy-Tower
This is a project to design and build a system that uses a combination of direct and indirect solar collection to generate electricity and store thermal energy in an economical, environmentally friendly, scalable, reliable, efficient and location independent manner using common construction materials.
The project is being managed with a similar methodology to Open Source Software Development and the ideas and contributions are being published openly on the Internet without an attempt to secure patents. The hope is that with an open philosophy that the project shows similar Rapid Application Development and success as Linux and other Open Source Software projects and provides a system that can meet future energy requirements in a sustainable manner.
The main focus of the design is to build a feasible renewable base load power station for moderate climates like Canada and the northern U.S.A., Asia and Europe where there is high solar insolation during the summer, but very cold temperatures and little daylight in winter.
The Solar Heat Pump Electrical Generation System (SHPEGS)
How it Works
A tower is built to allow large quantities of air to move across heat exchangers by natural convection due to buoyancy.
Solar thermal or deep geothermal heat is used to power a heat pump which moves a much larger amount of heat from the air.
Both the heat from the air and the heat powering the heat pump are stored in shallow heat storage.
The thermal storage is used to exploit the difference in temperature changes due to day time heating between the air and shallow underground, either day/night or seasonally. In effect this creates a local geothermal source and the low media transfer energy allows for an efficient geothermal power generation system. This source is reliable and may be used for base load electrical generation and structure heating.
In moderate climates where there is substantial differences in air temperature through day/night and/or seasonally, the system would function bi-directionally.
"Hot Air" cycle (ambient air warmer than ground)
A low boiling point fluid (ammonia) is expanded in the heat exchanger in the tower and where it boils (anhydrous ammonia boils at -33C) and expands. The ammonia is then absorbed into cool water. This aqueous ammonia solution is heated by solar thermal collectors or deep geothermal heat and the ammonia boils off under pressure. The ammonia vapor is condensed and the pressurized anhydrous ammonia is then returned to storage. Some of the heat is converted to electricity and the subsequent heat is stored. The cooled air falls in the tower creating wind and this energy is also captured in the wind turbines.
Step by step detail (PDF format)
Flow Animation (requires FlashPlayer)
"Cold Air" cycle (ambient air colder than ground)
The heat stored underground is used in a turbine very similar to existing geothermal systems. The turbine is air cooled with heat exchangers in the tower and the heat causes convection in the tower and this is also captured in the wind turbine and converted to electricity.
In high humidity tropical climates, the ambient air temperature remains relatively close to the shallow surface earth temperature and the temperature gradient would not make a bi-directional system feasible. The extraction of clean water from the humid air at a height is a major benefit of this system in a tropical location. A twin-tower in a "U" shaped system with a continual down and updraft air flow would be a design intended to dissipate as much heat as possible in the hot climate. The system would use large anhydrous ammonia storage to allow night operation and require large solar collectors to recover the ammonia in the day. During sunlight periods the solar collectors and ammonia storage would need to be large enough to allow sufficient ammonia to be recovered/re-pressurized to allow for continual operation. The system wouldn't use thermal storage and the ground would only be utilized as a heat sink to dissipate excess heat.
Many people have difficulty visualizing why this system in net energy positive, because when refrigerants (low boiling point fluids) are mentioned they lose the concept of the steam engine and start thinking about refrigerators and air conditioners. Refrigerators and Air Conditioners require energy (are not net energy positive) because they are moving heat from a cold area to a warmer one (like pumping water uphill), but this system is always moving heat from a warmer area to a colder (like water flowing downhill) and is energy positive.
In an arctic climate where there is access to medium temperature geothermal a much simpler system than existing low-boiling-point fluid steam turbines can be built with a convection tower. The major benefits of this system are simplicity potentially could have lower construction and maintenance cost than complicated low-gradient fluid turbine systems.
This system would perform well through the cold season and the temperature gradient from 70ºC geothermal to -30ºC ambient air allows for high efficiency. For this system to be efficient in a convection only system, the tower would need to be extremely tall.
Introduction of moisture to the air lowers density and increases buoyancy, but will probably cause snow and ice crystals to fall in the local area.
Very Basic Concepts
Warm humid air is less dense than cool dry air and this causes convection due to buoyancy
Water vapor is less dense and lighter than air (this is a little counter-intuitive for many people)
The heat from the air is "upgraded" by the ammonia system and the system output is all of the heat from the air plus all of the heat to move it
Geothermal, Biomass Coal or other waste heat may be substituted for or supplimented to the Solar Thermal Collection
When a liquid boils, it takes more heat than normal raising of it's temperature and it greatly expands in volume creating pressure
Boiling point increases with pressure
When the sun shines, the air warms up quicker than the earth (shallow underground)
At night or in winter, the air cools off quicker than the earth (shallow underground)
Heat moves from hot to cold with a force, when this happens some of the energy can be converted to mechanical energy
The Concepts in More Detail
When the temperature of the air is changed compared to the surrounding air, the density changes and it makes the air heavier or lighter than the surrounding air and this causes convection. Wind.
Matter that is more more dense takes more energy to change temperature than matter that is less dense. The temperature of the earth below the surface or in bodies of water changes temperature slower than the air does because they are more dense and they also take longer to cool off.
To "capture" mechanical (electrical) energy from heat, heat has to be moved from hot to cold. It doesn't matter where the heat is moving, but the mechanical energy captured is always a percentage of the heat that is moved based on the absolute temperature of the "cold" sink. The more heat that moves between matter and the larger the difference between the hot and cold sinks, the more mechanical (electrical) energy can be captured. An easy way to visualize this is by imagining a hydroelectric dam on a river. Some of the water may be used to generate power, but because the output of the dam is usually above sea level, you cannot use all of it. Getting mechanical energy from heat works the same way, it is always a percentage of the heat being moved and the amount of energy that can be converted is a function of the quantity and the difference in temperature between the hot and cold source. The difference between the cold sink and absolute zero determines the efficiency of the system. Water flows downhill with a force and heat moves from hot to cold with a force and both require energy to reverse the process.
During day/night or seasonal changes, there are substantial differences in temperature between the earth and the air. That difference in temperature can be moved from hot to cold and some of that energy can be used to generate electricity.
Except for a very small portion of the earth, the ocean (or ground) isn't always colder than the surrounding air. The air temperature in Western Canada swings from +30C to -30C, but the earth temperature a few meters below ground stays at around +3C. Just as much power can be generated from -30C air as +30C air.
Water freezes and the transport media has to have a lower freezing point than the coldest ambient air to have a location independent system.
If the thermal storage is either a natural or man-made underground system, it won't harm the environment. Denser materials like rock or metals will hold even more heat than water.
The system is base load electrical generation.
The solar energy collected is used to move a much larger amount of heat from the air.
The heat pump system can be powered from multiple sources (solar, geothermal or waste heat).
This system will be available in sub-zero temperatures and can generate as much power when it is really cold as when it is really hot.
Due to the reversible cycle, the energy stored or removed from the earth is used in the opposing cycle.
The system should be scalable from the single dwelling or remote equipment power source up to the MW grid project.
The system is "tuned". The more heat transferred through the heat pump, the more convection occurs. The more convection that occurs, the more heat transferred through the heat pump. The more heat that moves the more mechanical energy that can be "harvested" and converted to electricity.
The condensation on the cooling coils may be used to provide a clean domestic water source or for irrigation as a by-product during the air cooling cycle.
The system should operate in a wide range of climates with the limitation that there is sufficient solar heat above ground level and sufficient thermal transfer below ground level .
A rotating or finned air intake/output leveraging prevailing winds would increase performance and it should also improve system startup.
The system could be integrated with biomass methane production or with algae agriculture.
Actively "cooling" the pumps, turbines and generators and using the heat will make it very efficient. (contributed by Mark Smith, September 2006).
In colder climates where the ambient air temperature is below freezing for 6 months of the year, the system is really "renewable" because the amount of heat added and removed from the ground balances on an annual cycle.
In some locations there are natural geothermal heat sources at deeper levels that may be used in low solar isolation areas.
The physics of this design have not yet been in question, but the economics of the capital investment has. In calculating the economics of non-trivial renewable energy systems the traditional study using current market prices of goods and services is flawed. Our current economy is based on non-renewable energy and therefore it is a large portion of the "cost" of goods and services for common materials and construction. Eventually non-renewable energy systems run out of supply or cause damage to the ecosystem and the "cost" of damage to the environment is hidden for the short term comparison.
A fair evaluation of a non-trivial renewable energy system is energy input for materials, construction and maintenance versus energy output or EROEI. If a system can be constructed from common materials that will not be in short supply, the feasibility of the system is whether that system can produce enough renewable energy to construct a like system within a reasonable length of time. The initial capital cost is largely irrelevant if the energy output criteria is met and the system is a maintainable and a renewable energy source.
Of course, our economy currently is not based on renewable energy and until a substantial portion of our energy supply is met by truly renewable sources, real world economics are very important. The design criteria for this system allows for this by allowing for simple integration with other clean energy systems. The seasonal thermal storage may be used to heat buildings, ethanol fermentation or methane bioreactors. Biomass pyrolysis gas and methane can be burned in reciprocating or gas turbine engines and the heat output readily integrated. Biodiesel and Ethanol production facilities can also become more feasible with integration into this system.
Our current economy is based on finite resources. As an example, if an oil or gas well is drilled, there is an exploration cost and drilling cost. Eventually the well runs dry and again there is an exploration and drilling cost. This same problem is appearing with semiconductor supplies in Solar PV. As more finite resources are used it becomes more difficult and expensive to locate and collect and the economy continues in inflation. The constant increase in the price of fossil fuels also increases it's own exploration and extraction cost.
If a completely renewable system can be built from common materials and can produce enough renewable energy to build a like system within a reasonable length of time, it is feasible.
de Robert J. Rohatensky
(fragment of the original page)