Tommaso Toppi

Decarbonizing the building sector requires heat pumps to be increasingly retrofitted into existing residential buildings, where conventional hydronic systems often demand high supply temperatures. Propane is among the most promising low global warming refrigerants for this purpose, but its operation can be limited by high compressor discharge temperatures under severe conditions. This work experimentally investigates whether blending propane with pentane can mitigate this limitation and how a bypassable internal heat exchanger affects system performance. For this purpose, a heat pump prototype was built and tested with pure propane and 90/10 and 80/20 wt.% propane/pentane mixtures under space heating and domestic hot water conditions, with and without the internal heat exchanger. The results show that no single configuration was optimal over the whole operating envelope. The internal heat exchanger improved the performance when sufficient margin was available on the compressor discharge temperature. At the same time, pentane addition lowered compressor discharge temperature and became more favorable for larger heat-sink temperature variations, although it penalized the heating capacity. As a result, in space heating operation, pure propane remained preferable at lower sink temperature levels or when heating capacity was the dominant constraint, whereas the mixtures became beneficial mainly at higher temperature lifts when combined with the internal heat exchanger. In domestic hot water operation, where the sink-side temperature variation was larger, the mixtures combined with the internal heat exchanger showed the clearest benefits.

Heat and cold storage technologies are essential for increasing the use of intermittent renewable energy sources. Among them, thermochemical storage can offer higher energy density than sensible storage. This work experimentally investigates a single-bed silica gel/water adsorption system with a falling-film evaporator used as a cold storage unit, with specific attention to how the discharging phase can be controlled. Tests were carried out under different operating conditions by varying the chilled-water temperature and the temperature of the water cooling the adsorber to quantify the cooling energy released during adsorption. In addition, different parameters were evaluated to assess their potential in controlling the cooling-power profile during discharge: the opening of the valve between the evaporator and the adsorber, the cooling-water flow rate through the adsorber and the activation of the falling-film circulation pump. The measured cooling energy density ranged from 31 to 162 kWh/m3 for cooling-water temperatures between 25 and 40 °C and chilled-water temperatures between 10 and 20 °C. Among the tested control options, modulation of the valve between the evaporator and the adsorption bed proved to be the most effective strategy, as it reduces the initial power peak, smooths the release of stored energy and extends the discharge duration.

Increasing the temperature of waste heat is crucial to enable its recovery. Vapor compression heat pumps and absorption heat transformers are the two heat upgrade technologies most commonly used for this purpose. Heat pumps have the advantage of entirely recovering the waste heat and the disadvantage of requiring electricity as input. Heat transformers need a negligible amount of electricity but reject at part of the waste heat input at low temperature. Due to these differences, the choice between the two options depends on the application. In this work, the environmental and economic performance of heat pumps and heat transformers are compared in some relevant applications. Indications about the most suitable technology are provided based on the availability of the waste heat, of the CO₂ content of the electricity and of the electricity–gas price ratio. Heat pumps perform better when the waste heat availability is limited compared to the upgraded heat requirements and has a better environmental profile when the electricity has low carbon content. Heat transformer results are often economically convenient, especially when the availability of waste heat is large.

In this work, the performance of an innovative evaporator based on water falling film was investigated. The studied evaporator has been equipped with a recirculation system to maximise the wetted surface. Tests have been carried out in a lab-scale adsorption unit connected to a test bench recently realised at Politecnico di Milano labs for evaluating heat transfer performances under realistic operating conditions. Several ad/desorption cooling cycles were performed, setting different liquid refrigerant initial contents (0.9–1.5 kg), different chilled water inlet temperatures (7–20 °C) and flow rates (200–1000 L/h) and different adsorbent bed temperatures (25–30 °C). Evaporation performance has been determined in delivered cooling capacity. Moreover, the experimental data were used to calculate the overall evaporator heat transfer conductance (UA). Experiments showed how the heat duty peaks are mainly due to the thermal level of the chilled water that enters the evaporator, not the water content inside it because this value only affects the duration of the process. Instead, the UA value does not depend on the evaporator inlet chilled water temperature and initial mass content inside the evaporator. UA is 540–570 W/K for temperatures of chilled water entering the evaporator, equal to 10–20 °C, and mass of refrigerant of 0.9–1.5 kg.