Viability of an absorption solar cooling system in an isolated school in Mexico

The use of renewable energies, especially solar energy, as an external energy source for activating space cooling systems is a particularly suitable option, since energy needs related to air-conditioning increases when solar radiation levels are typically high. Moreover, systems driven by thermal energy are also very attractive in locations where access to electric power is limited or non-existent.

 

Within thermally-driven technologies, absorption cooling cycles are the most used option to take advantage of low temperature thermal energy, as they have the ability to generate cold with heat source temperatures from 75 °C. In addition, this technology has great maturity.

 

A recent article from Mexican researchers published in the International Journal of Refrigeration ^[1] presents the results of a simulation and initial experimentations of an absorption cooling system driven by solar thermal energy that has been set up in an elementary school in the community of Puertecitos, Baja California, Mexico. In this coastal village, isolated from the electricity grid, the ambient temperature can reach up to 50 °C, making air conditioning essential.

 

The simple-effect absorption cooling machine uses LiBr-H2O mixture as working fluid. The system has a cooling capacity of 35 kW and can be activated in a temperature range of 75–95 °C, thus suitable to be coupled to solar collectors. The collectors system consists of 110 m2 of evacuated tube solar collectors that produce the thermal energy needed to activate the chiller, while four cooling coils of 8.75 kW each cool the school classrooms with the chilled water produced.

 

The system also includes a thermal storage tank of 12 m³ to contain the hot fluid coming from the solar field and to supply the necessary energy to the cooling system in periods of zero or low solar radiation.

 

The cooling circuit of the machine has an evaporative cooling tower and tanks for the extra-water that are essential given the lack of water resources in the region.

 

In order to determine the optimum operational strategies and to ensure that the cooling needs of the four classrooms are met, a simulation study was conducted as a first stage before the project was completedThe results showed that the installed system is able to meet the cooling needs of the school with the simulated strategies. Yet for a stable operation, periods of time must be dedicated to the recovery of energy inside the thermal storage tank.

 

The cooling tower is indispensable for the operation of the chiller but represents a very high consumption of water, and its water supply may be an issue. To solve this problem, the possibility of using seawater in the cooling tower with a circuit independent of the water entering the chiller is being studied.

 

The total cost of the cooling system was USD 80,000, equivalent to USD 2285 USD/kW of cooling, which is a high specific cost compared to mechanical compression technologies. The absorption cooling machine corresponds to the most expensive equipment in the system with a price of USD 19,850.  However, the cost of this type of machines should decrease.

 

^[1] Aguilar-Jiménez J.A. et al. Optimum operational strategies for a solar absorption cooling system in an isolated school of Mexico, International Journal of Refrigeration, April 2020. Available in FRIDOC, free of charge for IIR members.