Yu Xie, Shi-jun Wu, Can-jun Yang, Generation of electricity from deep-sea hydrothermal vents with a thermoelectric converter, Applied Energy, Volume 164, 15 February 2016, Pages 620-627, ISSN 0306-2619, http://dx.doi.org/10.1016/j.apenergy.2015.12.036.
Subsurface ocean observations and monitoring of hydrothermal vents (figure 1, figure 2) can only be as good as the instruments used. A common obstacle with instruments is the life span of their power sources. Traditional power sources in marine science observation of hydrothermal vents include batteries or power cables. Both can be expensive; batteries, for instance, may require the use of a submersible. In addition, the use of power cables may not always be feasible.
An ideal solution is a power source that has a long life span, requires minimal maintenance, and can utilize the natural surroundings at hydrothermal vent systems. Power sources that already adhere to these parameters include piezoelectric energy, seawater batteries, hydroelectricity, benthic microbial fuel cells (MBFCs) and environmental fuel cells (EFCs). In the extreme environments of vent systems however, some of these power sources cannot withstand long-term observation periods. For example, batteries are dependent on the availability of oxygen and hydroelectric power requires fluid flow, both of which may be limited at hydrothermal vents. MBFCs and EFCs on the other hand are robust, long lasting, and in the case of EFCs, generate electricity by exploiting the oxidation-reduction potential between vent fluid and seawater; their limitation is that they produce power on the scale of micro to milli watts per cubic meter.
Exploration to improve the power harnessed from vent systems is a hot topic. Hydrothermal vents make ideal power sources because of the temperature gradient formed between the 400 degrees Celsius vent fluid, discharging at meters per second, and seawater. By utilizing the Seebeck effect (figure 3) scientists are able to use thermoelectric generators (TEGs) to measure thermoelectric voltage created from the temperature gradient. In reality there are many obstacles to TEGs: The extreme environment of high temperature, pressure, salinity and corrosive fluid limits the life span of TEGs, sulfide mineral deposition can cause TEGs to become defunct, typical chimney shape and rugged terrain of vent systems can be a challenge in the placement of big equipment, and because heat dissipates from the vent fluid immediately after leaving the vent it is difficult to capture. To overcome these challenges Xie et al. (2016) sought to design a thermoelectric converter (TEC) that harvests the hydrothermal energy through a heat pipe, and then uses TEGs to convert the heat into electrical energy. During field tests researchers used a power management system (PMS) to use the TEC to power an LED light bulb and data logger to record output current and voltage.
The TEC design can be viewed in figure 1 of the original article. The total weight is .4 kg, and it can withstand up to 45 MPa, which allows deployment to about 4500 meters. It includes a heat pipe with an evaporator and condenser (230mm length, 10mm diameter), four TEGs, four conduction blocks, a thermally insulated chamber, a heat dissipation shell, a protective shield, and an end cap. The heat pipe, chamber, shell, and cap are made of the strong and corrosive resistance titanium alloy (6Al4V). The blocks are made of thermally conductive aluminum (6061). The TEC Xie et al. (2016) designed was unique because the heat pipe is inserted directly in to the chimney, so the loss of heat from the fluid mixing with seawater and sulfide mineral build up inside the pipe are theoretically eliminated.
How it works
Thermal energy is transferred through the heat pipe to the TEGs, where it is converted into electrical energy. Conduction heat transfer forces the heat to the cold side of the TEGs to where there are conduction blocks, and then to the heat dissipation shell. The heat transfer between the shell and the seawater cools down the TEG, enhancing the temperature gradient. The protective shield protects the shell from vent fluids and keeps the TEG side temperature cool, also helping to enhance the gradient. Thermoelectric voltage is measured in response to the temperature gradient; when a temperature difference of at least 40 degrees Celsius is detected the data logger automatically turned on.
The TEG was field tested December 2014- January 2015 at the Dragon Flag Field along the Southwest Indian Ridge. It was deployed to 2765 meters with the assistance of submersible Jiaolong. The TEC was only deployed for 15 minutes due to limitation on bottom time for the submersible. Fluid at the vent field reached 379 degrees Celsius.
The TEC generation power was between 2.6-3.9 Watts during the field test. That is enough power to operate oceanographic sensors like turbidity meters, dissolved oxygen meters, pH sensors, conductivity meters, thermometers, and underwater cameras. A possible way to run instruments that require higher power would be to utilize batteries for energy storage and a PMS.
During the field test is it estimated that 188W of power were lost of the 433W (calculated) supplied from the vent. Possible heat losses from the fluid are leakage via air convection in the chamber and heat radiation.
Future work includes improving efficiency and testing for long-term observation capabilities.
Check out the original article to indulge in the thermodynamic equations Xia et al. (2016) used to conceptualize their idea and calculate their results.