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Solar Thermal (Solar Water Heating)
Solar water heating systems harness the heat from sunlight to produce
hot water. Domestic solar water heater usually comprises solar collectors
and a water tank. The solar collectors harness the heat solar radiation
to produce hot water, either directly or indirectly, which is stored
in the water tank for subsequent applications. A typical solar water-heating
system reduces the need for conventional water heating by about
two-thirds. It minimizes the expense of electricity or fossil fuel
to heat the water and reduces the associated environmental impacts.
Solar Water Heating System in San Diego, USA
Solar Water Heating Resource Potential
The technical performance of a solar water heater depends on the
water use load patterns (seasonal and daily) and the weather conditions.
A recent study found that a solar water heating installation with
a 4m2 solar collector and water tank volume of 250 to 270 L could
produce about 8,200 MJ/yr of thermal energy. The solar insolation
is 18,980 MJ/yr for the 4m2 collector, hence giving an efficiency
of 43% for the solar water heating installation, taking into account
of auxiliary power.
The results indicate that solar water heaters work effectively
in Hong Kong and that significant energy savings can be achieved.
Systems with smaller water storage tanks and smaller solar collector
arrays, for smaller apartments, would have slightly lower performance
than the 250 litre system, small tanks typically have higher loss
per unit storage volume. Conversely, larger systems as that would
be adopted in hospitals and hotels would be more efficient, as the
storage tanks could be as large as 3000 liters in these facilities.
Solar Collectors
The solar water heating collector is by far the most widespread
solar energy conversion device and there are several millions in
use around the world. It is a device in the solar water heating
system to harness solar radiation and convert it into useful heat.
Solar collectors can be categories into 2 major types based on design,
i.e. flat plate collectors and evacuated (vacuum) tube collectors,
with the latter further divided into glass-glass tube type, glass-metal
tube type.
Flat plate collectors
A flat plate solar collector contains a metal plate (called the
absorber made of copper, stainless steel or aluminum) employed in
an insulated rectangular box with a glass or plastic cover. The
metal plate is usually painted black in color to enhance heat absorption.
Air, water or fluid passing through the metal plate is heated by
solar radiation.
Evacuated tube solar collectors
1. Glass-glass tube type
The collectors contains parallel rows of transparent glass tubes.
Each tube consist of two glass tubes which are fused together at
one end. The inner tube is coated with a selective surface that
absorbs solar energy well but inhibits radiant heat loss. Air is
removed from the space between the two glass tubes to form a vacuum
to reduce heat loss. Heat is transferred to the inner fluid tube.
2. Glass-metal type
Evacuated tube with heat-pipe
Evacuated tube with heat pipe solar collector consists of a heat
pipe in a sealed vacuum glass tube. The heat pipe is filled with
a fluid and is bonded to a coated copper fin (absorber) to harness
solar radiation to generate heat. At the upper end of the heat pipe
is a condenser bulb where heat exchange takes place. The tubes are
mounted, with the condenser bulbs up, into a storage tank or a manifold.
Heat energy collected by the copper fin is used to vaporise the
heat pipe's working fluid, which rises into the condenser bulb.
Potable water from the recirculation loop flows through the water
tank or manifold and picks up the heat from the condenser bulbs.
The condensate then returns to the collector heat zone by gravity.
Solar Water Heating Systems
Solar water heating systems can be divided into 2 categories, the
open loop type and the close loop type, depends on whether the domestic
water is heated up directly or indirectly via a heat exchanger.
Open loop system
In an open loop system, domestic water passes through the collectors
will be heated up directly.
Close loop system
In a close loop system, domestic water is not heated up directly
by going through the solar collectors. Instead, fluids like Freon,
propylene glycol, distilled water are employed in the collectors
and circulating in a close loop. The fluid in the collector will
be heat up and pass through a heat exchanger, where heat is transferred
to domestic water.
Sizing a solar water heating system
By simple calculation
[Web facility for calculating the size of a solar water heating
system required]
By computer simulation
此網頁的超連結有可能引向由第三方發行或營運的網站。Software
simulation: RETScreen International
Resource potential
Technical guidelines
此網頁的超連結有可能引向由第三方發行或營運的網站。Solar
Water Heating - Technical Guidelines and Specification
Solar Water Heating - Directories
You can get the list of directories from the following web site:
Solar Water Heating - Local and International
Standards
Solar Water Heating - Rules and Regulations
Building/Structural Requirements
Waterworks Standard Requirements
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