Page 132 - Design of Solar Thermal Power Plants

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS 119

3.2 HELIOSTAT FIELD EFFICIENCY ANALYSIS
FOR POWER PLANTS

Under conditions corresponding to the design point, the energy flow of
various system parts shall be balanced and calculated by applying the
calculation method for the Yanqing (Badaling) 1 MW power plant as
shown in Table 3.1. According to Table 3.1, solar concentration losses in
the heliostat field rank second-largest among all losses and are caused
primarily by cosine and interception errors. Cosine loss can be deter-
mined from the relationship between solar position and the concentrating
field. Software currently available for calculating concentrating field
efficiency for a solar tower power plant includes WinDELSOL, heliostat
optical code (HOC) [18,19], and Fiat Lux.
The optical efficiency of the solar tower concentrator is important to the
thermal performance of the entire solar tower collector. The aperture
plane of a cavity receiver, and the (inner or external) absorbing surface of
any central receiver, are key interfaces of energy flux. So it is necessary to
simulate and analyze the concentrated time-changing solar flux density
distributions on the flat or curved receiving surface of the collector with
the main optical errors considered. The transient concentrated solar flux
on the receiving surface is the superimposition of the flux density dis-
tributions of all normal working heliostats in the field.

3.2.1 Brief Introduction to Heliostat Optical Code for Solar
Towers
HOC is a solar tower optics code developed by the Institute of Elec-
trical Engineering, Chinese Academy of Sciences (IEE-CAS) that can be
used for the layout and optimization design of heliostat fields and for
optical performance analysis of partial and full heliostat fields on receiver
apertures. HOC was developed with MATLAB and can be used for both
cavity and external cylinder receivers. HOC is based on new backward
ray tracing (BRT) and shading-and-blocking methods to simulate the
transient concentrated solar flux on the receiving surface for a solar tower
power plant [18,19].

3.2.2 Algorithmic Principles of Heliostat Optical Code

HOC uses the BRT method combined with the lumped effective solar
cone to simulate the flux density map on the receiving surface. For BRT,
bundles of rays are launched at the receiving-surface points of interest,
strike directly on the valid cell centers among the uniformly sampled
mirror cell centers in the mirror surface of the heliostats, and are then