Environmental Engineering Reference
In-Depth Information
either be glazed or unglazed. In general, air collectors are mostly applied if the end-users
have a demand for hot air, space heat, dry agricultural products, or to condition the
indoor air (air cooling). At present, air heating systems are mainly designed to directly
use the air for space heating. However, the opportunity for this application depends
directly on the market share of air heating systems, which is low in most countries.
A niche market is given by preheating of ventilation air for large volume buildings
(stores, sport halls, schools and other commercial buildings) where temperatures in the
range of 15 to 25
◦
C are desirable. With the very same air systems, hot water preparation
is often possible through an air/water heat exchanger, which is generally done during
the summer season in order to increase the overall performance of the system.
The application of air as a heat transport medium compared to liquid, has signif-
icant advantages along with few inevitable disadvantages. Using air as a medium of
heat transport avoids the damage caused by leakages, freezing or boiling. The disad-
vantages that brought up are the low heat transfer efficiency and high volume transfer
demand due to the lower heat conductivity and density. It also loses more heat energy
if leakage happens.
Tonui et al. (2007) carried out an experimental study on air-cooled PV/T solar
collectors in which a few low cost performance improvements were introduced. Both
water and air have been used for PV cooling through a thermal unit attached to the
back of the PV module. Compared to water, air is preferred due to minimal use of
material and low operating cost despite its poor thermo-physical properties. The study
has investigated the performance of two low cost heat extraction improvement mod-
ifications in the channel such as the use of a thin flat metal sheet suspended at the
middle or finned back wall of an air channel in the PV/T air configuration. A theo-
retical model was developed and validated against experimental data, where a good
agreement between the predicted results and measured data were achieved. The val-
idated model was then used to study the impact of various design parameters such
as the channel depth, channel length and mass flow rate on electrical and thermal
efficiency, PV cooling and pressure drop. The study had confirmed that the suggested
modifications positively improve the performance of the PV/T air system.
As the heat transfer in the air cooled PV/T system is much more critical than in the
liquid cooled PV/T system, it is important to model the heat transfer properly. Sopian
et al. (1996) presented a performance analysis of single-pass and double-pass PV/T air
systems. The performance of single-pass and double-pass combined photovoltaicther-
mal collectors are analyzed with steady-state models, with air as the working fluid.
The performances of the two types of combined photovoltaicthermal collectors were
compared. The results show that the new design, the double-pass photovoltaicthermal
collector, has superior performance over the conventional design.
For a flow through a tube or duct, entrance-effect plays an important role in
the heat transfer. Eicker (2003) presented an overview of entrance-effect heat transfer
relations for air-collectors, showing a variation of about 10% on the average Nusselt
number when integrated over the entrance length and reported that for a sufficiently
wide channel, the hydraulic diameter should be twice the channel height. Hegazy
(2000) analyzed four types of air PV/T model design including single glazed collectors
with air flow over (Model I) or below (Model II) the absorber; with air flow on both
sides of the absorber in a single pass (Model III); and a double pass model (Model
IV) as shown is Figure 5.2.2. The effects of air specific flow rate and the selectivity
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