The effect of two operating parameters, the regeneration temperature and the ambient temperature, on the performance of two-bed, closed-cycle solid-sorption heat pumps is investigated. The results show that increasing the regeneration temperature can improve both the COP and cooling capacity, and the effect on cooling capacity is more significant than the COP. Increasing the regeneration temperature from 180°C to 260°C, the cooling capacity increases by 50 percent and the COP improves 20 percent. When the ambient temperature drops, the system performance increases significantly. When the ambient temperature decreases by 10°C from the design point of 35°C, the COP and cooling capacity increases by 50 percent and 40 percent, respectively. Also, the bed cycling speed should be increased to achieve the optimum system performance when the regeneration temperature is higher or the ambient temperature becomes lower.
A rotary dehumidifier consists of a rotating porous matrix made of a desiccant with mechanically supporting materials. The dehumidification performance of a rotary dehumidifier wheel depends on its rotational speed, the sorption properties of the desiccant, the heat and mass transfer characteristics of the matrix, and the size of the dehumidifier. The effect of the rotational speed on the dehumidification performance of a rotary dehumidifier has been investigated by Zheng, Worek, and Novosel (1993). This paper extends that previous work and investigates the effects of desiccant sorption properties, the heat and mass transfer characteristics, and the size of the rotary dehumidifier on the dehumidification performance. The results show that the using desiccant materials in a rotary dehumidifier with different adsorption characteristics results in a wide variation in dehumidification performance. However, the maximum performance of a rotary dehumidifier occurs for a desiccant material having an isotherm shape that can be characterized to have a separation factor of 0.07. Also, as the desiccant moisture uptake increases, the dehumidifier performance also increases. However, the performance improvement for a desiccant matrix having a maximum moisture uptake of larger than 0.25 by weight is not significant. The heat and mass transfer properties and the size of a rotary dehumidifier are characterized by the number of transfer units NTU. Generally, the larger the NTU, the better dehumidification performance. However, similar to the maximum moisture uptake, when the NTU is larger than 12, the performance will not improve significantly. Also, the dehumidifier with the most favorable adsorption characteristic has a slowest rotational speed, which results in a lower power requirements to rotate the desiccant wheel and smaller carry-over losses.
The effect of design and operating parameters on the performance of closed-cycle, twobed sorption heat pump systems were investigated. The parameters studied in this paper included the effects of bed switching frequency (i.e., the switching speed), the sorbent bed NTU, the thermal resistance within the sorbent, the contact resistance between the sorbent and the tube wall of the heat transfer fluid, the fraction of inert mass within the sorbent bed heat exchanger, and the amount of fluid resident within the heat exchanger. The results show that the performance of a sorption heat pump system is extremely sensitive to the switching speed. In fact, the value of the switching speed that optimizes the COP is different from the value that optimizes the overall cooling capacity. Such a performance characteristic allows the design ofmultispeed sorbent bed heat pumps to be operated in such a way to follow the load while still maximizing the system COP. However, operation of the system at switching speeds away from the optimum values can cause a dramatic deterioration in system performance.
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