Low spread geothermal is the production of energy using pressures created by changes in temperature where the working fluid does not undergo a phase change. Low spread geothermal with the Ristau Motor uses a closed loop system to take advantage of the Carnot Cycle. Low spread geothermal does not require hot pockets in the earth. Instead, Low spread Geothermal uses the constant temperature of the ground as a base which can act as either a heat sink or a heat collector. An additional benefit is that constant ground temperatures are much more accessible being found near the surface (roughly 15-20′ deep) instead of miles below as is the case with traditional Geothermal.
Geothermal Energy Production
Most power plants regardless of type need steam to generate electricity. The steam rotates a turbine that activates a generator, which produces electricity. Many power plants still use fossil fuels to boil water for steam. Geothermal power plants, however, use steam produced from reservoirs of hot water found a couple of miles or more below the Earth’s surface. Prior to the invention of the Ristau Motor three types of geothermal power plants were possible: dry steam, flash steam(enhanced), and binary cycle.
Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant, where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers.
Flash steam (enhanced) power plants are the most common. They use geothermal reservoirs of water with temperatures greater than 360°F (182°C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir.
Low-Temperature Geothermal
Low-temperature geothermal energy is defined as heat obtained from the geothermal fluid in the ground at temperatures of 300°F (150°C) or less. These resources are typically used in direct use applications, such as district heating, greenhouses, fisheries, mineral recovery, and industrial process heating. However, some low-temperature resources can be harnessed to generate electricity using binary cycle electricity generating technology.
Binary Cycle Power Production
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 400°F) 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 closed-loop systems and virtually nothing (except water vapor) is emitted to the atmosphere.
Low Spread Geothermal
Low spread geothermal is the production of energy using pressures created by temperature changes where the working fluid does not undergo a phase change. Low spread geothermal with the Ristau Motor uses a closed loop system to take advantage of the Carnot Cycle. Low spread geothermal does not require hot pockets in the earth. Instead, Low spread Geothermal uses the constant temperature of the ground as a base which can act as either a heat sink or a heat collector.
A heat exchanger in the ground as well as a second exchanger exposed to temperatures either higher or lower than the constant base create a temperature spread/imbalance that can be used to make mechanical work. The mechanical work is then converted into electrical energy (illustrated below). Temperatures as a whole do not matter as long as the spread between the exchangers is sufficient to cycle the processors.
In the illustration above, a heat collector (red loop) heats a working fluid (air) causing it to expand and create pressure. The heat source can be ambient air temperature, focused solar, biomass, steam etc. The pressure created by the heated gas causes a primary circulator (the primary Ristau Motor) to cycle. A portion of the mechanical energy is used to drive an electrical generator while the rest of the energy is used to cycle a re-circulator (another Ristau Motor) which allows the process to be repeated. After the working gas passes through the primary circulator, the processed gas vents into a heat sink (blue loop) which can be any colder body such as ground temperature or a body of water where it cools and contracts. The re-circulating Ristau Motor driven by the primary circulator is used to put the cooler more dense gas back into the heat collector where it again expands driving another cycle.
Low temperature spreads are available around the wolrd making it the most abundant and available energy source on earth. While this is common knowledge in scientific circles, the pressures created by normal environmental temperature changes are so low that existing processors could not use them to create useful energy. The invention of the Ristau Motor makes it possible.
To illustrate the potential energy available from low temperature spreads, consider the graph below. The graph below uses Gay-Lussac’s Law to calculate the change in pressure of an ideal gas in a closed system. The chart has been converted from the usual Kelvin and Torr to Fahrenheit and PSI to make it easier to relate to our every day environment. The temperatures charted below range from 32 degrees to 120 to represent temperatures we may experience on any given day.
Temperature (in Fahrenheit) is represented by the X axis and pressure (in PSI) is represented by the Y axis. You can see from the chart that with a temperature change of the 88 degrees illustrated by the chart, that the initial pressure of a closed system starting at standard atmospheric pressure increases only 2.63 PSI. This change is very small, even pitiful. That is a benefit when you consider that the tires on your vehicle don’t go flat in cold weather and explode during warm weather, but it is not much to work with in terms of energy production. Luckily the pressure created by temperature change is compounded by magnitude of how much initial pressure in the system. This is mathematically expressed in the combined gas law where P=Pressure, V=Volume and T=Temperature. The same point can be made this way; The more gun powder you put in the bomb, the bigger the bang. By increasing the initial pressure in the system we can expect better results over the same temperature spread. We see this below where the X axis is temperature and the Y axis is pressure:
You can see that by pressurizing the system to 100psi, the temperature change of the same 88 degrees yields a pressure increase of 17.9 which is a much better result than the paltry 2.63 of the non pressurized system. Now we are working with pressures that can actually do some work! For further illustration, lets double the pressure to an initial pressure of 200psi leaving the other factors the same.
Of course, as dictated by the combined gas law, doubling the initial pressure from 100psi to 200psi prior to exposing it to the same 88 degree increase has effectively doubled the yield. The result is a pressure increase of 35.8psi. That’s roughly the air pressure in the tires of your vehicle! The 3 charts have been combined below for more direct comparison.
These are still relatively low pressures. Consider that the copper plumbing in your house can probably handle pressures over 1000psi. It is important to take into consideration that the above calculations were done using an ideal gas. Real world working fluids may react differently. Some working fluids are susceptible to undergoing a phase change (change from liquid to solid to gas to plasma) while others may react differently under different pressures which would show a curved graph rather than a linear one. The attributes of real world working fluids can be a benefit or detriment depending on the application.
When using low spread geothermal for electrical output 3 factors need to be considered: Temperature change, initial pressure and volume. We have already considered the first two in relation to each other.
1) Temperature Change: Knowing the temperature change you are working with whether it be daily highs and lows compared to ground temperature or if you are using a heat source like reflective lenses or a boiler allows you to calculate the pressure produced in your system.
2) Initial Pressure: As illustrated above, the higher the initial pressure, the more molecules you have in the system to perform work.
3) Volume: The volume of the heat exchangers is very important because it dictates how fast you can harness energy from the system. A change in temperature will give the same pressure regardless of volume but the heat collector and heat sink must be large enough for the working fluid to take on the temperature of the surrounding material before reaching the next processor. If the heat dissipation or collection rate is low, the exchanger(s) must be larger to achieve the desired output. If the heat dissipation or collection rate is quick the exchanger(s) can be smaller (but do not have to be unless there are space restraints).
By manipulating each of the above factors the desired output can be reached. If one of the controlling factors is low the other two can be increased to compensate.