Resumen de Solar-driven hybrid refrigeration systems based on thermochemical processes
The field of refrigeration is affected either directly or indirectly by all forecasted concerns on global warming, population increase, price rise and uncertain availability of fossil fuels. Renewable energy sources are a promising response, and among these, solar energy has yield satisfying results in refrigeration. A shortcoming of this source is its intermittence, which is classically solved with auxiliary primary energy coming from fossil fuels. Another solution is to increase the solar coverage through sensible or latent thermal storage, with some drawbacks related to thermal losses.
Thermochemical processes are based on an interaction (either sorption or reaction) between a gas and a sorbent or a reactive salt, which is usually in solid state. These systems can be applied to refrigeration and can store energy. Given the nature of the process, the heat supplied to the system is not affected by thermal losses and can be stored for relatively long amounts of time. In addition, the higher energy density of these systems (up to 10-fold and 4-fold that of sensible and latent storage, respectively) reduces the volume of equipment per unit mass of refrigerant. The fact that these systems store liquefied refrigerant, which can be stored indefinitely at ambient temperature and immediately released for cold production, is also worth mentioning.
A major drawback of thermochemical systems is the intermittence of the process itself, because the sorbent pair or reactive salt are in solid state. Unless this intermittence is solved, it is a limitation for applications where there is a continuous demand of cold.
To overcome this limitation, this thesis proposes hybrid refrigeration systems that revolve around thermochemical processes. The novelty lies in the hybridization with well-known, state-of-the-art refrigeration systems. The resulting hybrid systems are expected to operate with solar energy, store energy, and have a small degree of autonomy (a few hours within a daily operating cycle). In addition, some of the components are shared by the two processes, making the overall system compact.
Two solar-based energy sources were targeted: on one hand, low-grade solar thermal energy (< 120 ºC), which can be utilized for single-effect absorption refrigeration cycles; on the other hand, solar-PV energy, which can be applied for compression refrigeration cycles. As a result, this thesis proposes two solar-driven hybrid refrigeration systems with thermochemical processes as the central piece. In both systems, the refrigerant fluid, the condenser and the evaporator are shared by the subsystems, for the sake of compactness.
The first one is an absorption/thermochemical (ABS/TCH) hybrid system driven by low-grade solar thermal energy (< 120 ºC). The configuration, components, pressure and temperature levels and operating modes are discussed. A preliminary performance estimation with some ammonia-based and water-based working pairs finds the NH3/NaSCN and NH3/BaCl2 pairs suitable for the ABS and TCH subsystems, respectively. A preliminary evaluation of the hybrid system in a simulated application shows that, with increasing mass of stored refrigerant and area of solar collector field, solar coverage increases from that of solar absorption without storage to a limit value (< 1), while the global COP decreases from that of the ABS subsystem (around 0.7) to that of the TCH subsystem (around 0.3).
The second hybrid system consists of compression/thermochemical (COMP/TCH) refrigeration and is driven by solar-PV electricity or grid-distributed electricity. It also requires a heat source, but thanks to the compressor, it can utilize low-grade sources (as low as 30 ºC), which is interesting for waste heat utilization. After system definition, operating modes description and working pair selection, the study focuses on simulating the compression-assisted decomposition phase in the TCH subsystem. A 2-front quasi-steady reaction model is presented which allows to account for heat and mass transfer limitations. This model is used to preliminarily study the influence of some operating conditions and design parameters on the system’s advancement degree - reaction time (X-t) curve.
The 2-front reaction model is later validated through an experimental study of the non-assisted and compression-assisted decomposition phases of the TCH subsystem. An experimental setup was built similar to the COMP/TCH hybrid system, with all measurements focused on the reactor/compressor interaction. The experiments showed activation temperature reduction of the TCH process and yielded enough experimental data for model validation. The values of permeability and thermal conductivity of the reactive salt were adjusted for a non-assisted decomposition phase, and after adjustment, model predictions were confronted to the remaining decomposition phase experiments, as well as synthesis phase and, finally, compression-assisted decomposition phase. It is concluded that the adjusted model predicts the X-t curve with acceptable accuracy for almost all experiments, with some discrepancies in the compression-assisted decomposition phase.
The systems proposed in this doctoral systems are interesting in their concept, and the first results seem promising for further research.
Keywords: Solar refrigeration, hybrid systems, thermochemical heat transformer, performance simulation, experimental study.
