KEY BREAKTHROUGHS

Key Breakthroughs of 247Solar Technology

The key breakthroughs of the 247Solar concept include two critical modifications to the then existing approach to Brayton Power Towers that were being advanced in similar fashion about 10years ago by the German Aerospace Center (DLR) and Israel’s Weizmann Institute of Science. The commercial viability of both systems was severely constrained by two important factors based on their “high-pressure” designs. The first is power size. The nature of their systems makes it technically difficult and rather expensive to achieve power sizes greater than 100kWe. The second constraint is lack of thermal storage for off-sun operation. This means that such systems must compete against PV, which also cannot operate off-sun and which has quickly dropped in price.

The high-pressure system configuration is shown below. It is shown with a thermal storage system, which can be achieved using multiple high-pressure solar receivers. Note, however, that the entire system must operate at high-pressure, not just the solar receiver but also all ducting, thermal storage, blowers and valves/dampers. High-pressure requires that all of these components are “pressure vessels,” which adds significant costs and risks.

Of greatest significance in this regard is that the diameter of the glass aperture of the solar receiver cannot be very large or it will burst, just 60-80 cm (22-29 inches), because it must contain enormous pressures, up to 7 or more atmospheres. This small diameter limits the receiver’s thermal output to approximately 350 kWth, large enough to power a 100 kWe turbine with nothing left over to store as heat for nighttime operation.

A high-pressure Brayton cycle power tower system. (Thermal storage is included here for comparison with the low-pressure system in the next Figure.) Note that the high-pressure air from the turbine’s compressor and recuperator passes through the solar receiver en route to the turbine. It seems simple enough but the result is that the receiver must be designed as a “pressure vessel.” This in turn means that the diameter of the glass aperture of the receiver cannot be very large or it will burst, just 60-80 cm (22-29 inches), because it must contain enormous pressures, up to 7 or more atmospheres. This small diameter limits the receiver’s thermal output to approximately 350 kWth, large enough to power a 100kWe turbine with nothing left over to store as heat for nighttime operation.

The low-pressure system is shown below.

The principal breakthrough leading to the 247Solar concept was the introduction of a high-temperature heat exchanger into the system configuration. This heat exchanger transfers the heat from the exit air of the solar receiver to the compressed air, avoiding the passage of high-pressure air through the receiver. With this approach, ambient (near zero) pressure air from the turbine’s exit passes through the receiver. No longer having to contain high-pressure air, the aperture of the solar receiver can be a larger diameter without bursting. In fact, the diameter of the prototype 247Solar Receiver is about 2 m (6+ ft), producing 7 times more heat than the high-pressure receivers. For the initial 247Solar Plant, this means powering 300kWe during the day while simultaneously storing enough heat to operate at full power 10-15 hours at night. It is likely that the diameter of the aperture glass in future 247Solar Receivers could be 4 m or more, increasing the power output of the system to 2MWe or more.


A LOW-pressure Brayton cycle power tower system. The principal breakthrough leading to the 247Solar concept was the introduction of a high-temperature heat exchanger into the system configuration. This heat exchanger transfers the heat from the exit air of the solar receiver to the compressed air, avoiding the passage of high-pressure air through the receiver. With this approach, ambient (near zero) pressure air from the turbine’s exit passes through the receiver. No longer having to contain high-pressure air, the aperture of the solar receiver can be a larger diameter without bursting. In fact, the diameter of the prototype 247Solar Receiver is about 2 m (6+ ft), producing 7-8 times more heat than the high-pressure receivers.