Utility Plants and Renewable Sources - Energy Conversion and Management - page 30

Cogeneration plants (see Chap. 9 ) , that is, plants in which either the coincident
generation of necessary heat and mechanical or electric power occurs, or which
produce power by recovering low-level heat from process, may reach an overall
efficiency ranging from 60 to 85 %, depending on the type of cogeneration plant.
This is a considerable primary energy saving, generally accompanied by a signifi-
cant energy cost saving, more or less important depending on the local regulations
and tariffs.
The main renewable sources of energy are solar, wind, geothermal, hydraulic
energy, and energy from waste. Most of these sources are inadequate to cover the
request of a single site, factory, or building, because they yield only low specific
power in standard conditions while the current capital and operating costs are rather
high. Nevertheless, in spite of these limitations, renewable sources should be
considered in the light of local conditions and efforts always made to exploit them.

4.3 Renewable Sources: Solar Energy

The solar energy reaching the earth surface can be used to produce hot water or to
produce electric energy by means of photovoltaic cells. Typical values of the rate of
the solar energy are 300-1,000 W/m 2 depending on the latitude, time of day, and
atmospheric conditions.

4.3.1 Hot Water from Solar Energy

Water can be heated by solar collectors which collect a fraction of the incident
sunlight ranging between 90 and 20 % depending on the number and type of glazing
and on operating parameters such as rate of insulation, ambient air temperature, and
input cooling fluid temperature.

With average values of 300-1,000 W/m 2 of solar energy rate, a standard
collector may yield an actual specific power ranging from200 to 600W/m 2
to produce hot water at 50-60 C (122-140 F) with a difference of 30 C
(48 F) between the average internal collector temperature and ambient
temperature.

Heat losses from a collector are generally defined by a coefficient (W/m 2

K)
which varies from 7 with a single glazed flat plate to 2.5 in the case of a double one
with selective surfaces (which have a selective coating such as black chrome to
reduce the emission of energy, particularly at high temperatures). The solar energy,
when converted into hot water or hot air, must be stored to match the end user
demand throughout the day.