The scientists from Taibah University in Saudi Arabia and Benha University in Egypt have developed a new solar power system capable of simultaneous electricity, heat, and cooling generation. Such systems are called trigeneration systems: a single energy source is used for multiple purposes.
The system is based on a tower with heliostats – mirrors capturing sunlight and directing it to a central receiver at the top. The researchers’ key engineering innovation is special spiral-shaped tubes inside the receiver which are equipped with internal fins. A heat transfer fluid, Syltherm 800 silicone oil, circulates through them. Syltherm 800 is high-temperature-resistant and is able to effectively transfer heat without decomposing.
Modeling showed that the optimal fin height is 2 mm, and the helical pitch is 42 mm. At a solar radiation intensity of 1000 W/m², this configuration increases the outlet oil temperature by 39.4% compared to smooth tubes. This effect is achieved by increasing turbulence and creating secondary flows within the spiral channels.
The resulting heat is used in a combined cycle consisting of two parts: a Kalina cycle power unit efficiently converting low-grade heat into electricity, and an absorption refrigeration unit using an ammonia and water mixture. This combination allows excess heat not to be lost, but to be used for cold and hot water generation.
Calculations have shown that adding of a refrigeration circuit increases the overall energy efficiency of the system by 23.78% and energy efficiency by 14.55% which means that the system loses less of the energy that could be converted into useful work.
With an increase in solar radiation from 450 to 1000 W/m², the total useful power of the unit increases from 144.8 to 202.5 kW. Electricity generation increases from 9.24 to 11.33 kW, heat from 14.18 to 19.1 kW, and cooling from 5.57 to 7.31 kW. Overall system energy efficiency increases from 48% to 54.5%, and energy efficiency – from 35.9% to 42.25%, meaning losses are reduced and more energy is used for useful work.
At the same time, the analysis revealed where the system loses the most energy. The main losses are in the central receiver, accounting for 22.45% of all unavoidable energy losses. Next is the heliostat array (11.67%), the superheater (6.45%), and the mixture separator (5.47%).
Despite this, compared to the similar solar trigeneration systems, the new design appears competitive and more efficient by a number of respects. And most importantly, it runs entirely on renewable energy and helps reduce CO₂ emissions by replacing traditional heating and cooling systems.
