New Adsorptive Cycle “Heat from Cold” for Amplification of the Ambient Heat: Adsorbent Synthesis and Testing Results

Adsorptive transformation of heat is an emerging technology for heating/cooling that is especially promising for low temperature heat sources. Recently, a new cycle for amplification of low-grade ambient heat, called “Heat from Cold” or HeCol, has been suggested. The HeCol cycle uses two natural thermostats, namely a water basin and the ambient air, both being at low temperature (about and below zero), as heat source and heat sink, respectively, to produce the useful heat at higher temperature. This paper addresses the synthesis, characterization of the adsorbents suitable for HeCol cycles, and the results of testing lab-scale HeCol unit. Firstly, we consider the thermodynamic requirements to an adsorbent optimal for several HeCol cycles, characterized by different working temperatures: TL of the heat sink, TM of the heat source, and TH of the useful heat, and methanol as working fluid. Then, we analyze the adsorption equilibrium of novel sorbents based on lithium chloride inside various porous matrixes, bearing in mind the formulated requirements. Finally, the feasibility of the HeCol cycle was explored on a lab-scale prototype loaded with one of the composite sorbents. It was shown that the adsorbents with an S-shaped adsorption isotherm are beneficial for the HeCol cycles, because they can be regenerated under “milder” operating conditions (at higher temperature TL) and exchange larger mass of methanol in the cycle. The methanol adsorption equilibrium of the composite sorbents LiCl/matrix is described by stepped adsorption isotherms. The sorbents can be regenerated at moderate TL = -20°С and TM=20-25°С with methanol uptake variation of 0.43-1.60 g/g. The feasibility of the HeCol cycle was demonstrated at the lab-scale prototype, loaded with the LiCl/silica composite. It was shown that the useful heat can be produced at TH=33-51°С suitable for warm floor systems, and the maximum specific heating power reached 6-8 kW/kg of the adsorbent. This attractive performance clearly demonstrates the feasibility of the HeCol cycle driven by low-grade heat to produce the useful heat suitable for heating.