Our Solar ORC technology has three main components: the parabolic troughs, the organic Rankine cycle (ORC) engine, and the electrical control system. All system components have been designed and tested with off-grid applications in mind, resulting in a technology that has a relatively simple design, is versatile, scalabile, and cost-effective, and is distinguished as a reliable, affordable, sustainable, distributed renewable energy system for developing countries.
Solar energy is collected by an array of parabolic troughs, each of which focuses incoming sunlight onto a pipe located at the trough focal point. The pipes are linked together to form a continuous loop through which a heat transfer fluid (like glycol, the anti-freeze fluid used in automotive radiators) circulates. As the fluid is pumped through the array, it is heated by the sun's rays, concentrated by the troughs onto the pipe, and reaches up to 150º C. The heat transfer fluid then passes through a heat exchanger where it transfers its heat to the working fluid (a refrigerant like those used in refrigerators or air conditioner units) of the ORC. The thermal fluid exits the heat exchanger at a much cooler temperature (around 100º C), ready to start another circuit through the troughs to absorb more of the sun's energy. These concentrators are constructed mainly using steel, copper piping, and aluminum reflective sheeting. Assembly is straight-forward, requiring welding, pipe joining, and simple fabrication capabilities.
The heat transferred to the working fluid in the heat exchanger is then used to drive the organic Rankine cycle (ORC) engine, a novel scaled-down version of the Rankine cycle historically used in Megawatt-scale solar thermal, coal, or natural gas-fired power plants. This closed-cycle engine has four stages, shown in the diagram above (bottom left). First as the working fluid is heated in the heat exchanger, it vaporizes to form a pressurized gas. This gas moves through a series of "expanders" (e.g., a turbine, scroll expander, or other positive displacement machine), causing them to spin (like air that drives a pinwheel). These expanders are coupled to electrical generators (typically a DC or induction motor run in reverse) to create electricity that can be used to charge a bank of batteries. After the gas has passed through the expanders, the working fluid is run through a condenser (a heat exchanger like an automotive radiator or air conditioner condenser) to transfer any remaining heat to the surrounding air and cause the fluid to re-condense into a liquid. It is then returned to the boiler via a pump. Our ORC engine is constructed using standard mass manufactured parts (HVAC scroll compressors and condensers, off-the-shelf motors and generators, some automotive pumps, typical refrigerant piping) that are high volume, low cost and ubiquitous. This practice reduces the cost of the system and increases availability of initial materials and replacement parts, making local manufacture, use of, and servicing of this system possible.
Optimization of the system is achieved through a unique autonomous control system that orchestrates the energy inputs and outputs to balance solar resources and the customer's needs while maximizing efficiency. This is made possible via a combination of sensor readings (temperature, pressure, flow rate, solar insolation level, etc.) and vetted control logic, managed locally via an embedded controller. Although currently system data is logged only locally, the STG team is working to upgrade systems for use with the GPRS (3G cellular) network so that Solar ORC outputs can be monitored in real time.