Japan, located on the Ring of Fire, is known for its high amount of geothermal energy. Some of the most visited areas within Japan are the hot springs that come from superheated water deep beneath the surface. However, these hot springs are culturally important to the Japanese, and since these hot spring areas are protected by national parks, 80% of the geothermal energy in Japan is inaccessible. As the need for energy increased, Japan turned to nuclear power, which works, but poses health risks if a meltdown ever happened, such as the Fukushima disaster. Since Japan is close to multiple fault lines, the risk of another nuclear disaster is high. In order to satisfy the need for clean energy that is not nuclear, and to leave the natural parks intact, we wanted to build a smaller geothermal plant that has high energy output, but much smaller than traditional geothermal power plant.
One of our group members spent six months in Japan on a mission trip and fell in love with the country. His knowledge of the local culture and customs of the nation, particularly around the area of Sendai, guided our developmental process so that our final project accounted for cultural differences between how Americans and Japanese citizens perceive the expansion of geothermal power. While such moves towards alternative power sources are lauded in the United States, our group member noted that many Japanese citizens are wary of geothermal plant expansion. In Japanese culture, natural hot springs and baths are considered sacred and beneficial for long lasting health. As a result, many in Japan feel that the expansion of large, centralized geothermal plants pose a threat to these precious sites. Thus, we had to account for this cultural difference when designing our proposal for geothermal power production; it’s the primary reason we opted for a decentralized approach to geothermal power production, rather than the standard model of creating a single power production facility.
What it does
Our project uses Japan's vast geothermal resources in order to wean its dependence off nuclear power and towards a more reliable, less dangerous source of energy. Project Hot Spot utilizes geothermal energy, yet not in the traditional method of steam and turbines. Instead, Project Hot Spot utilizes high-efficiency thermoelectric materials. Thermoelectric materials are compounds that, when exposed to a temperature differential, experience a voltage. The amount of voltage per Kelvin, or the Seebeck coefficient, can change depending on the material, and depends on atomic arrangement and charge carrier concentration. Up until very recently, thermoelectric semiconductors did not transform thermal energy into electrical energy well enough to make this solution viable. The invention of semiconductors that have a high enough ZT, or thermoelectric efficiency (above 2.5), have only come around in the last 3 years. Tin selenide has one of the largest ZT's of any other thermoelectric material, at about 2.7 at 800 K. By using thermoelectric generator (TEG), or chip, designs already in use and replacing the couplers with tin selenide, output wattage increased by a factor of 4 per generator, giving about 20 watts of power per 40mm x 40mm x 3mm chip. Our invented solution, named The Stack, uses the naturally-occurring steam and water-cooled heat sinks to create a temperature differential of around 300 K. The Stack is made from 22 drawers, each holding about 272 TEG's. Each drawer is sandwiched between a steam wall at about 600 K and a water wall at about 300 K, and utilizes modular “click” technology that allows for rapid and easy maintenance of each drawer. Each stack, at full capacity and optimum efficiency, can generate about 120 kW, and with 100 stacks, a power station can produce 10 MW per day. With 28 substations each with 100 stacks, we can power the entire population of Sendai.
Each Stack would be made with 2.7mm thick P235GH Steel Vessel, which is made to withstand high pressures. The inside of each wall would contain aerogel, which has an extremely low thermal conductivity and a high durability, properly insulating each wall while having the strength to handle the constant high pressures and temperature of steam. Geothermal steam is usually extracted at about 160 ℃ and 6 atm. Since we need the temperature at 327 ℃, we need to increase the pressure to 8.31 atm, and to correct for heat loss, the steam will be compressed to 10 atm. The well will be the first place of compression with a conical shape, getting smaller the higher the steam rises from the bottom of the well. The well walls will also be made out of P235GH steel bessel as well, which can withstand temperatures of 425 ℃. The well needs to be 2000m deep in order to reach the proper temperatures for steam extraction. The second compression step is a piston right before the steam goes into the steam wall, increasing it to the perfect temperature for optimal power output.
Each substation, with its 100 stacks, would all fit into an area of 1000 square meters of disturbed land, or 10,764 square feet. For comparison, traditional geothermal power stations disturb approximately 441,000 square meters. The well that will be drilled is only 10 cm in diameter, and with the invention of safer and more sustainable drilling practices, as well as the small size for the well, the impact to the surrounding environment will be minimal. Within the 1000 square meter size estimate for the substation, that includes all the necessary space for steam storage, pipes, and tubing for the entire substation.
How we built it
We created models of each of the constituent parts, including the TEG, the steam wall, the water wall with appropriate heat sink blades, and the drawer that houses all of the TEG’s. Both the steam wall and the water wall have holes for input and output valves, as well as to scale representations. We created the TEG from schematics obtained from II-IV Marlow, and used that design with different coupler materials (https://www.marlow.com/products/power-generators/thermoelectric-generator-teg-modules).
We chose the city of Sendai because of its large population and closeness to geothermal hot spots. The locations was determined from GIS technology and the use of map overlays to determine the area that would give the highest return on investment (ROI). We set ROI potential as areas in Japan that had the greatest population density and close proximity to ideal geothermal resources; ideal geothermal resources were defined as thermal hot spots that reached between 175-200॰C and were up to 2000m below ground. After developing our proposed power substations, we determined and plotted 28 ideal locations based on population density for each substation in order to provide adequate energy to the city and the surrounding areas.
Challenges we ran into
At one point, we could not find the right equations to calculate output voltage and output wattage because we lacked the proper knowledge of grid current for integration of each substation. However, we did find another equation that did not require current, and used that equation based on the number of chips and the output power of each chip.
Another problem we ran into was finalizing the final dimensions of the Stack. With each passing iteration on SolidWorks, the dimensions changed and we had to fix the efficiency of each of Stack, and change the final plans for the layout of the each substation.
Accomplishments that we're proud of
We are extremely proud of our ability to develop CAD models of not only the thermoelectric generators, which we dubbed “Chips”, but also thermopiles, which we dubbed “Stacks.” We were able to show the individual components and their integration through an exploded view diagram.
Furthermore, we are proud of our use of open source GIS software to identify ideal test locations for our thermoelectric power substations. We were able to identify a location, the Japanese city of Sendai, that had access to geothermal potential and had a population which could use the energy harvested from those hot spots.
We are also proud of developing such a feasible solution to the problem that Japan faces. Not only does this pull Japan back from nuclear energy, but it also establishes a cheaper, more minimally invasive, renewable energy source.
What we learned
Through this project, we learned more about the plausibility of geothermal energy harvesting and the various types of geothermal harvesting that currently exist. Furthermore, we learned about thermoelectric materials and their effectiveness in turning temperature gradients into power that can be amplified by connecting generators in series. We also learned GIS software and how it can be effectively used to identify needs of an area.
What's next for Project Hot Spot
The next step of the research is to create an actual TEG made with tin selenide and test the efficiency with different temperature differentials. With these actual extracted results, we can better determine the cost and feasibility of Project Hot Spot. After that experiment, we would go into integration of a steam wall, finding the exact dimensions and materials needed to endure constant high temperature and pressure while being cost-effective. After the steam wall, the integration of a two-drawer steam wall with proper TEG would be tested to determine the efficiency of the conducting metals, develop and improve the design for the project, and investigate how to control the temperature differential within the TEG. After that, it would go into real world prototyping and testing.