Basic Research Needs for Solar Energy Utilization

THREE TYPES OF CONCENTRATED SOLAR “THERMAL” SYSTEMS

cold water

hot water

direct & diffused sunlight

process

heat

Low-temperature solar thermal systems.

This system does not involve sunlight

concentration, or occasionally may employ

low-concentration (C ≤ 2). This system is

suitable for home use where it usually heats

hot water for direct use or home heating.

Concentrated photovoltaic (CPV) system. Concentrated

photovoltaics, although not a solar thermal process, is included in

this survey because of the utilization of concentrators.

direct sunlight

optical concentrator

concentrated

sunlight

electricity

photovoltaic

power conversion

direct sunlight

optical concentrator

concentrated

sunlight

cold input

hot output

solar to thermal

power conversion heat to electricity

or

heat to chemical potential

power conversion

High-temperature solar thermal systems.

Such systems require optical concentration

of direct sunlight at a ratio varying from about

20 to several thousands, depending on the

specific process and system involved.

Concentrated solar thermal systems are

large megawatt(electric) systems that can be

used for electricity generation or fuel

production.

Central Receiver Systems. Central receiver systems contain an array of Fresnel reflectors

(heliostats) with two axes of rotation. The common focus is stationary, located on a solar tower

(Figure 14b). The two-axis tracking enables a higher concentration ratio and the higher operating

temperatures and power conversion efficiency than those of the line focus configuration.

However, as the system size increases, the optical efficiency (the ratio of sunlight capture to

incident sunlight) declines. Thus, system optimization is required.

Two 10-MWe facilities are situated in the United States near Barstow, California, and one

2.5-MWe facility is in Almaria, Spain. Present estimates of large-scale (>50-MW) facility costs

are about $3/W (Sargent & Lundy 2003; Stoddard et al. 2005). A recent study (Pitz-Paal et al.

2005) indicates that the new developments discussed here should lead to a cost reduction of at

least $0.5/W. Materials development aimed at high-temperature performance would impact

efficiency.