Microchannel Adsorption Heat Pump

Position – Graduate Research Assistant

Host – Sustainable Thermal Systems Laboratory 

PI – Dr. Srinivas Garimella

Duration – February 2016 – December 2016

Motivation 

The annual USA primary energy consumption is 104 EJ, of which nearly 14.2 EJ is consumed by heating, cooling and air conditioning (HVAC) applications, while an additional 3.23 EJ is used for water heating. The HVAC sector is dominated primarily by the vapor compression cycle, which involves cyclic compression, condensation, expansion, and evaporation of a refrigerant. Because of their reliability, compactness, and scalability, vapor compression systems have been used extensively in cooling and heating applications for nearly a century. However, there are two primary drawbacks of using vapor compression systems. First, they are driven by electricity. This contributes to the electricity consumption in the residential and commercial sector, which currently draws 75.5% of the generated electricity in the U.S. Second, the conventional refrigerants used in the vapor compression cycles have the potential to cause harm to the environment. Although modern cycles use HFCs, such as R-134a or R‑22, which have negligible ozone depletion potential (ODP), they have significant global warming potential (GWP). Hence, widespread efforts are being directed toward the investigation of refrigerants with low ODP as well as GWP, and also with low flammability and toxicity.

Thermally-activated cooling systems address the important challenges mentioned above. The greatest advantage of thermally driven technologies is that waste heat from other applications such as power plants and engine exhaust suffices for their operation, with minimal additional electricity consumption. Among the available thermally driven cooling technologies, adsorption heat pumps offer advantages including: (a) the use of only thermal energy for operation, (b) the absence of moving parts, enhancing durability (c) the option to use non-toxic, non-flammable working fluids, such as water, (d) the ability to use low-temperature heat sources (e) reduction of peak electricity demand, (f) use as cooling or heating systems by reversing the refrigerant flow direction. These advantages can be realized using microchannels for constructing the adsorbent bed for the systems, as the conventional embodiments of adsorption heat pumps restrict their applicability due to some key design limitations. The adsorption and desorption cycle times for such systems can be up to a few hours depending on the system size and working pair, making them impractical for many applications. Nevertheless, adsorption gas separation systems have recently been shown to exhibit exceptional process capacities with cycle times as short as 20 s , if packed beds are replaced by coated microchannels.

Objectives 

  1. Incorporate adsorbent-coated microchannels in adsorbent beds of heat pumps and conduct component-level fluid flow, heat and mass transfer modeling of a representative proof-of-concept unit for a cooling capacity of 300 W at 5°C.
  2. Determine optimum channel geometry, and heat source and sink temperatures through parametric studies.
  3. Compare the cyclic steady state performance of the heat pump with the literature and assert the advantages.

Design

The design considered in the present work consists of several rows microchannels coated with silica gel as an adsorbent material, lined with several rows of heat transfer fluid (HTF) channels used for manipulating the temperature/thermodynamic state of the adsorbent as shown in Figure a. Such a design can be made by using metal shims, etching channels on them, coating adsorbent on one set of them and then stacking them in an alternate fashion as shown in Figure b.

Adsorbent bed design for heat pump/refrigeration system design.

A complete cycle as shown in the Figure below was simulated in MATLAB to predict the refrigerant cycling ability, in turn the cooling capacity. During the bed operation, first hot water at 90°C enters the HTF channels heating the adsorbent and releasing the water refrigerant in vapor form. As a result the pressure in the adsorbent channel increases, opening the one-way valve to the condenser, pushing the water vapor in the condenser, where it loses enthalpy to become liquid water.

Heat pump system schematic showing the water refrigerant flow.

During the next stage, cold water at 35°C enters the HTF channels to cool the adsorbent bed. This process decreases the adsorbent temperature and as a result the adsorbent adsorbs the residual water vapor in the channel, lowering the bed pressure. This opens the one-way valve between  the evaporator and bed. Because of the pressure differential created between the two ends of the evaporator, liquid water from the condenser passes through the evaporator picking up heat from the space to be cooled.

This is standard adsorption cooling cycle, while the difference between the conventional and this design is related to the time required for the operation. It is possible to complete the desorption and the condensation stage in less than 20 s, whereas the cooling can continue for up to 600 s. This asymmetric staged operation is the key finding from this work. The figure below shows the refrigerant mass in the adsorbent bed every cycle, where it can be clearly seen that the microchannel-based adsorbent bed can reject the refrigerant in less than 20 s, while taking over 600 s to regain the cyclic steady state value.

More parametric studies on the geometry resulted in the COP values as high as 0.36 with total bed volume of as small as 7 liters.

Cyclic variation of adsorbent bed refrigerant mass.