Environmental Engineering Reference
In-Depth Information
for high temperature applications, the space between the glass cover tube and the
receiver is evacuated to further reduce convective heat loss from the receiver tube and
thereby increase the performance of the collector. The total receiver tube length of
PTCs is usually from 25 m to 150 m.
In this chapter a detailed thermal model of the receiver of the collector is pre-
sented. Many researchers have published studies of energy models of parabolic trough
collectors. The most important ones are the following.
Edenburn (1976) predicted the efficiencies for focusing collectors which consist
of a cylindrical parabolic reflector and a collector tube surrounded by a transparent
envelope and which heat a fluid flowing through the collector tube. These efficiencies
have been predicted using analytical heat transfer methods. The analysis considers
visible radiation transfer, IR radiation exchange, conductive and convective losses and
energy transferred to a fluid flowing through the collector tube. The collector may have
a tilted north-south axis, an east-west axis or it may fully track the sun and geometric
parameters associated with tracking the sun are considered. Both evacuated and non-
evacuated cases are considered and the predicted results are in excellent agreement with
collector performances measured using Sandia Laboratories' collector test facility.
Clark (1982) analyzed the effects of design and manufacturing parameters that
influence the thermal and economic performance of parabolic trough receivers. This is
achieved by an identification of the principal design factors that influence the technical
performance of a parabolic trough concentrator and which relate directly to design and
manufacturing decisions. These factors include spectral-directional reflectivity of the
mirror system, the mirror-receiver tube intercept factor, the incident angle modifier and
absorptivity-transmissivity product of the receiver tube and cover tube, the end loss
factor and a factor describing the effect of tracking errors and receiver tube misalign-
ment. Each of these factors has been quantified in terms of design and manufacturing
tolerances and associated performance degradation. Other design considerations that
relate to thermal loss from the receiver tube are low emissivity coatings, evacuation and
anti-reflection coating. The analysis of energy costs using the parabolic trough concen-
trator determines both the break-even, current metered cost of energy and the annual
cash flow over periods of investment ranging from 5 to 15 yr. The economic factors
include investment tax credit, energy equipment tax credit, income tax bracket, cost
of auxiliary system, foundations and controls, cost of collector, costs of maintenance
and taxes, costs of fuel, cost of capital, general inflation rate and fuel escalation rate.
Karimi et al. (1986) applied a piecewise two-dimensional model of the receiver, in
which the receiver of the collector is divided into longitudinal and isothermal nodal
sections as shown in Figure 6.1.3, performed by considering the circumferential vari-
ation of solar flux and applying the principle of energy balance to the glazing and
receiver nodes.
Heidemann et al. (1992) studied the temperature field in the absorber tube of a
direct steam generating parabolic trough collector. Steady-state and transient operating
conditions are considered. They formulated a two dimensional heat transfer model for
calculating the absorber wall temperature of a DSG collector under both conditions.
A universal program was developed for solving the two-dimensional transient tem-
perature field using a modular nodal point library. The temperature field is extremely
asymmetric due to the variation of the heat transfer coefficient at the inner surface and
the solar irradiation at the outer surface of the absorber tube. High temperature peaks
Search WWH ::
Custom Search