When evaluating concentrated solar power (CSP) solutions, buyers often tend to focus on considerations such as: How many kilowatts of electricity will it produce? How much space will it require? What is the cost and timeline for installation and the time required for payback?
While these are undoubtedly all critical elements, there is another important -yet commonly overlooked- question that you should be asking: Does the solution also produce a usable thermal energy by-product?
CSP applications that produce usable heat energy as a by-product offer a unique advantage over those that only generate electricity. The heat can be used for a wide variety of applications and potentially serve as an additional source of revenue.
How does it work?
CSP involves the use of reflected and concentrated solar heat. This is used to heat an energy transfer medium that can then convert the heat energy to kinetic energy in a turbine. The vast majority of CSP technologies use water as the heat transfer medium. They heat the water, under pressure, to between 350⁰C and 550⁰C to produce steam. This steam drives a steam turbine, which in turn spins a generator to produce electricity. The steam condenses to water after passing through the turbine and the now unpressurized water still contains a large amount of heat energy. However, because steam turbine-driven plants are large scale (there are no small steam turbines), they require a lot of space and are located in remote areas. This means the heat energy that leaves the turbine is impossible to use in a commercially viable fashion.
By contrast, new CSP technology developed by AORA employs a gas turbine rather than a steam turbine. A gas turbine is powered by pressurized hot air. In the AORA Tulip, air is heated by reflected and concentrated solar heat to a temperature of 1000⁰C. Gas turbines are available in small sizes, and the output of a single AORA Tulip is 100kW. Compare this to 50MW (500 times as much as 100kW), which is the case of the smallest commercially available steam turbine.
This capability allows the AORA Tulip to be built close to the user. The proximity allows for the potential use of the residual heat available, after the turbine has taken its kinetic energy and the heat in the exhaust has been diverted to preheat the incoming air. This residual heat is produced at the rate of 88m³ per minute at a temperature of 250⁰C.
It is generally more efficient to transport heat energy in a liquid medium than a gas, such as air. Therefore, most applications using the heat energy will involve a “heat exchanger” that transfers the heat from the exhaust air to circulating water.
7 practical applications of CSP generated exhaust heat
- Heating water: Water can be heated for either domestic or institutional purposes for use in places like dormitories or hospitals. This may include hot water for cooking, bathing, cleaning, and more. While this is perhaps the most intuitive usage, it is arguably one of the most important applications, and the savings are easily calculated – the cost of heating water by use of an alternative energy source.
- Industrial processes: Hot water can also be used to clean industrial equipment and machinery. Some sectors, including beverage bottling plants for example, require very large quantities of hot water for both production and maintenance. Steam can also be produced by using a pressurized heat exchanger that will allow the water to be heated above 100⁰C. Again, the savings is in the avoidance of the cost of heating the water or steam.
- Cooling: Absorption chilling can be used for food refrigeration (which will maintain a temperature of about 4 -7 degrees Celsius). In many areas of the world including India and most of sub-Saharan Africa, refrigeration is a real game-changer. It allows women to work outside of the home and not have to prepare meals from scratch each day. Refrigeration can also be used for medication.
- Agriculture: Greenhouse plants and crops can benefit from heat at night and cooling during the day to maintain a set temperature throughout the year. Additionally the CO2 produced by the combustion of biogas or other fuel to produce power at night can be ducted to the greenhouse to help the plants grow.
- Accelerating biogas production: Biogas processing is more efficient with higher temperatures. Secondary heat can be used to speed up the process of the digestion tank turning waste into fuel. This is significant because the biogas can quickly become its own source of renewable electricity or heat, or be further processed to provide a source of renewable fuel.
- Space heating or cooling: Of course the free heat energy can also be used for space heating in homes, factories, dormitories, hospitals, etc. Conversely, with the use of absorption chillers, the same heat can provide cooling.
- Generate even more electricity: If only electricity is required and there are no uses for the thermal energy for heating or cooling, a Rankine cycle turbine (which can use lower temperature heat to generate power) can be used to increase the power output of the overall solar solution.
Of course, buyers are always looking to maximize the benefits of their investment. While a CHP (Combined Heat and Power) solution may not meet the specific requirements of every site and region, there are many locations that could truly benefit from this combination, providing two kinds of energy at a combined lower price.
So, the next time you evaluate a new solar investment, be sure to inquire about other potential benefits beyond just producing electricity.
Written by Zev Rosenzweig, CEO of AORA Solar