(399e) Evaluation of Polyol Ester Oils As Absorbents in a Hybrid Absorption Refrigeration Cycle

For remote grid-independent operations, electricity production is one of the largest consumers of fuel, and typically the largest consumer of electrical power is climate control (i.e., air conditioning). Therefore, the efficiency of climate control systems dramatically impacts energy requirements. While the vapor-compression refrigeration cycle used for climate control is relatively efficient, with a COP_cycle of 2.5, the conversion of thermal energy from fuel to the electrical energy that powers the vapor-compression cycle is typically only 27% efficient. The remaining 73% of the energy is lost as waste heat through the engine exhaust, engine cooling system, and resistive losses in the electrical system. Calculating its efficiency based on the engine's thermal energy rather than the mechanical energy entering the compressor yields a COP_heat of 0.68.

A novel hybrid-absorption refrigeration cycle was proposed that utilizes electricity and waste heat concurrently to improve the overall efficiency of the electric generator and vapor-compression refrigeration cycle scenario. This hybrid cycle combines a vapor-absorption cycle run on waste heat parallel to a vapor-compression cycle powered by electricity. In this scenario, an electric (e.g., diesel) generator provides waste heat and electricity. This research focused on measuring the vapor-liquid equilibrium (VLE) for potential working fluids, modeling the resulting solubility data with an equation of state, and thermodynamic simulation to evaluate the efficacy of the resulting hybrid absorption cycle. Refrigerants R134a and R410 were chosen as the potential refrigerant fluids for this application, and polyol ester (POE) compressor lubricant was chosen as the absorbent for the vapor-absorption branch because it will already be present in the vapor-compression branch of the system, avoiding any complications from cross-contamination between the vapor-compression and vapor-absorption branches.

The VLE measurements in this research were performed using an isochoric technique similar to Wahlstrom and Vamling (1997). The refrigerant was dosed at a known temperature, pressure, and volume and then introduced to a known amount of POE. The refrigerant vapor in the system headspace was then recalculated with the final temperature, pressure, and volume. The difference between the initial and final mass was assumed to have been absorbed into the POE sample.

The resulting bubble-point data was then modeled with the Soave-Redlich-Kwong (SRK) equation of state (EOS), as demonstrated by Yokozeki et al. (2005). The five SRK fitting parameters were determined with a least squares method using VLE data from the present research and prior published data. The properties of the pure fluids and mixtures within the cycles, including the enthalpy and composition of each phase present, were then calculated using the SRK equation of state and the parameters found in this study. The performance of the chosen working fluids was then simulated in three different cycles, and the resulting COPs were compared across a range of ambient temperatures. The simulation of the hybrid absorption cycle was performed with the same temperature dependencies used in the previous simulations. A hybrid cycle, with a total refrigeration capacity of 1 ton (tonR, 3.5 kW, 12,000 BTU/hr), was simulated as being paired with an electric generator that provides the required electrical energy and expelled waste heat. The simulation assumed that 30% of the fuel energy entering the electric generator was converted to mechanical energy, while 70% exited as waste heat. The model then assumed that 60% of the waste heat was recovered from the engine exhaust and coolant. The simulation was performed across a set of operating conditions, varying the amount of power the generator provides, resulting in varying amounts of waste heat for the hybrid absorption cycle. These operating conditions included zero waste heat (vapor compression only), generating only the power required to run the hybrid absorption cycle (Balanced), and several cases where the generator produces additional electricity for other demands beyond the hybrid cycle.

The simulation results showed that the efficiency of the cycle is improved with the hybrid configuration. As expected, the improvement in COP is directly related to the amount of available waste heat. Using refrigerant R134a and the POE oil as an absorbent at an ambient temperature of 30^oC (86^oF), the hybrid absorption cycle was 8.7% more efficient in balanced operation and nearly 74% more when the generator was providing an additional 8kW of electricity production, producing more waste heat. Based on this study, it would seem thermodynamically feasible to create a hybrid-absorption cycle capable of harnessing waste heat to aid the refrigeration cycle and significantly increase overall efficiency. This cycle would be particularly beneficial in deployed military applications, where the delivered fuel cost can exceed $15 per gallon, resulting in a short payback period. However, other factors, such as increased size and weight, must also be considered.

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