Air condensed water ammonia absorption chillers are attractive for solar cooling because of low parasitic consumption and high thermal COP. The main drawback is the need for medium temperature heat (200 °C above ambient temperature). Heat storage at medium temperature is expensive and, fortunately, not necessary in solar cooling. Instead, cool can be stored in order to compensate for the intermittent availability of solar radiation. Based on this concept, two demonstration plants have been erected within the MEDISCO project. The first one makes use of a high concentration ratio (about 25) linear focusing Fresnel reflector, with highly insulated (vacuumed) absorber tubes. On the contrary, the second one relies on a low concentration ratio (16.5) PTC, with glass covered atmospheric absorber tubes. The apparent contrast existing between the two solar technologies was the motivation for this study. Is the low concentration ratio PTC coupled with atmospheric absorber tubes an optimal choice from an efficiency point of view, at the temperature of interest for solar cooling? If so, would this still be the optimal choice with high performing vacuumed tubes available on the market nowadays? To answer these questions, optical and thermal characteristics of PTC are reviewed and a design optimization method for PTC is proposed. A PTC design optimization method is proposed along with the analytical formulation of collector efficiency and intercept factor. According to proposed method, PTC design for temperature difference above ambient in the order of 200 °C will be sensitive to the insulation level of absorber tube. Market available PTC with non insulated tube are optimally designed for the temperature range of interest. However, substituting non evacuated tubes with evacuated ones would lead to a decrease in optimal concentration ratio from about 15 to about 11 and an increase in efficiency of about 30%. Assuming constant output and same absorber diameter, this means a decrease in aperture area in the range 20% - 25% and a small increase in absorber length of about 4%.
Parabolic trough Collector Design and Optimization for high temperature lift Solar Cooling
APRILE, MARCELLO;MOTTA, MARIO;PISTOCCHINI, LORENZO
2009-01-01
Abstract
Air condensed water ammonia absorption chillers are attractive for solar cooling because of low parasitic consumption and high thermal COP. The main drawback is the need for medium temperature heat (200 °C above ambient temperature). Heat storage at medium temperature is expensive and, fortunately, not necessary in solar cooling. Instead, cool can be stored in order to compensate for the intermittent availability of solar radiation. Based on this concept, two demonstration plants have been erected within the MEDISCO project. The first one makes use of a high concentration ratio (about 25) linear focusing Fresnel reflector, with highly insulated (vacuumed) absorber tubes. On the contrary, the second one relies on a low concentration ratio (16.5) PTC, with glass covered atmospheric absorber tubes. The apparent contrast existing between the two solar technologies was the motivation for this study. Is the low concentration ratio PTC coupled with atmospheric absorber tubes an optimal choice from an efficiency point of view, at the temperature of interest for solar cooling? If so, would this still be the optimal choice with high performing vacuumed tubes available on the market nowadays? To answer these questions, optical and thermal characteristics of PTC are reviewed and a design optimization method for PTC is proposed. A PTC design optimization method is proposed along with the analytical formulation of collector efficiency and intercept factor. According to proposed method, PTC design for temperature difference above ambient in the order of 200 °C will be sensitive to the insulation level of absorber tube. Market available PTC with non insulated tube are optimally designed for the temperature range of interest. However, substituting non evacuated tubes with evacuated ones would lead to a decrease in optimal concentration ratio from about 15 to about 11 and an increase in efficiency of about 30%. Assuming constant output and same absorber diameter, this means a decrease in aperture area in the range 20% - 25% and a small increase in absorber length of about 4%.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
