New backing for Aberdeen-based Exnics will see its Hot Rings system developed into a thermoelectric power generator
Getting power to the seabed is notoriously tricky. Typically facilities are powered from the surface via tieback, which is expensive. Reducing this financial burden could lower the overall costs of production by a sizable margin. The solution, as many see it, could be to site power generation facilities on the seabed itself, but this brings with it a host of new challenges.
One advantage is that many subsea pipes are already carrying hot hydrocarbons. A typical well will produce up to several megawatts of heat, yet this energy is not used, and is usually allowed to dissipate into the surrounding water. One Aberdeen-based firm, Exnics, wants to harness this heat and use it to power subsea equipment. Its so-called Hot Ring technology takes the form of a collar which is attached to the exterior of a production spool. These pieces are comprised of solid-state ceramic, contain no moving parts and generate no emissions. The collar clamps around the outside of a standard nominal pipe section and will passively scavenge the energy otherwise lost to sea. As the heat passes through the device, some of the energy is captured via a semiconductor material and converted into a DC electrical current. Suitable for pipelines of between 4 inches and 12 inches (102-305 mm) in diameter, Exnics says that these devices can be deployed and connected at substantially lower cost than subsea power cables and, because they require no input fuel, will also reduce operating costs. Over a 20-year operating life, that equates to significant savings. In addition, because the system is built with chemically and galvanically inert materials, neither will they corrode – reducing the potential for inspection and maintenance.
Watts up The system is a viable solution for flowlines operating at 50°C or over, up to a maximum of 200°C. Power generation and efficiency increase with temperature, and with the diameter of pipe. According to Exnics’ computational fluid dynamics simulations, one ring fitted to a 10-inch (254-mm) pipe operating at 200°C can produce as much as 70 W. Hot Rings are designed to track the maximum power by adjusting to changes in the environment. By monitoring heat flux and the corresponding changes in the internal resistance of the semiconductor over its lifetime, Exnics says the Hot Ring can achieve over 90% electrical efficiency in converting harvested heat to power. They are also scalable. According to Exnics, a “limitless” number of Hot Rings can be connected together to cater for a variety of power demands, producing a suitable voltage and current to operate subsea controls, sensors, actuators and even pumps. In particular, the company says that the system works best when used in combination with a Li-ion battery. These will help to regulate DC output to prevent overcharging, but a train of Hot Rings could provide a float charge for larger subsea batteries, allowing generation system to meet high intermittent loads and to maintain power during cold shutdown periods. Rated to 3,000m, the system would be of particular benefit to deepwater and ultra-deepwater infrastructure, and for infrastructure at high-pressure, high-temperature (HPHT) wells.
TERS response Building on the capabilities of Hot Rings has led Exnics to adapt the technology for use at subsea wells. Locating a thermal energy recovery system (TERS) here not only capitalises on the maximum possible heat output, but also places the generator close to much of the load equipment. It is this endeavour which will take up much of Exnics’ time over the next year. The company has recently secured new development funding from the Oil & Gas Innovation Centre (OGIC). As part of the group’s thirty-first project agreement, Exnics will benefit from a share of GBP230,000 (US$290,000) in investment, spread across four new industry technology projects. The new backing will see Exnics work with Heriot-Watt University to develop a Thermoelectric Generator (TEG) to support its Hot Rings system. A bespoke TEG will be developed which will work specifically with the thermoelectric clamp. The goal is to improve the performance of the product in future deployments and to increase the range of applications where power from waste heat can be used. OGIC chief executive Ian Phillips commented: “The Exnics project is an example of where further research is being done to enhance new technology, which is already being tested on the market. This technology has the potential to offer cost reductions to companies and a number of operators have already noted interest in the technology.”