Other areas whose development could lead to substantial cost reduction are means to increase the
concentration ratio and improve the cooling of the cells. Costs of HCPV systems will continue to
drop as the cell efficiency is increased and various other development objectives described in the
PRD document are reached. Installed costs of $2–3/W can be reached within five to ten years.
THERMOELECTRICS
Thermoelectric energy conversion technology exploits the thermal energy of electrons for energy
conversion between heat and electricity, including power generation, refrigeration, and heat
pumping. It is based on the Peltier effect and the Seebeck effect. Because the working fluids are
electrons and holes in solids, thermoelectric energy conversion technologies are environmentally
benign. A thermoelectric power generator has a maximum efficiency given by
hc m
h mch
TT 1ZT 1
T 1ZT T/T− +−
η=
++. (8)
A refrigerator has a maximum coefficient of performance (COP) given by
c mhc
hc m
T 1ZT T/T
COP
TT 1ZT 1+−
=
− ++, (9)
where
Th, Tc = temperatures at the hot and cold sides
Tm = mean temperature
Z = thermoelectric figure of merit that is a combination of the Seebeck
coefficient S, the electrical conductivity σ, and the thermal conductivity k:
S^2
Z
kσ
=Because Z has a unit of inverse Kelvin, ZT together is called a nondimensional figure of merit.
These expressions show that thermoelectric devices with a high ZT value have an efficiency that
approaches the Carnot limit. Thus, the key to thermoelectric technology is finding materials with
a large ZT. Because of the conflicting requirements on materials properties, high-ZT materials
are elusive. Materials with reasonable ZT are usually heavily doped semiconductors and some
semimetals. For making efficient devices, both n-type and p-type materials with comparable ZT
are needed. The ZT values of a material are temperature dependent; they usually peak at certain
temperature and drop off at higher temperatures. Thus, refrigeration and power generation often
require different materials, depending on the operational temperature range.