There are a number of solar how water system types on the market. This article explores a number of common system type and describes the differences between. If you're still trying to figure out which kind of system is for you, this article is for you. The types of solar systems discussed are pumped (both direct and indirect), thermosiphon and integral collector storage.
Direct Pumped Systems
Differential controller operated system
The direct pumped system, illustrated in Figure 1,
has one or more solar energy collectors installed on the roof and
a storage tank somewhere below, usually in a garage or utility
room. A pump circulates the water from the tank up to the collector
and back again. This is called a direct (or open loop) system because
the sun's heat is transferred directly to the potable water circulating
through the collector tubing and storage tank; no anti-freeze solution
or heat exchanger is involved.
This system has a differential controller
that senses temperature differences between water leaving the
solar collector and the coldest water in the storage tank. When
the water in the collector is about 15-20° F warmer
than the water in the tank, the pump is turned on by the controller.
When the temperature difference drops to about 3-5° F, the
pump is turned off.
In this way, the water always gains heat from
the collector when the pump operates.
A flush-type freeze protection
valve installed near the collector provides freeze protection.
Whenever temperatures approach freezing, the valve opens to let
warm water flow through the collector.
The collector should also
allow for manual draining by closing the isolation valves (located
above the storage tank) and opening the drain valves.
recirculation is another means of freeze protection. When the water
in the collector reaches a temperature near freezing, the controller
turns the pump on for a few minutes to warm the collector with
water from the tank.
Figure 1. Typical direct pumped system
The system shown in Figure 2 , differs from other direct pumped
systems in that the energy to power the pump is provided by a
photovoltaic (PV) panel. The PV panel converts sunlight into
electricity, which in turn drives the direct current (dc) pump.
In this way, water flows through the collector only when the
sun is shining.
The dc pump and PV panel must be suitably matched
to ensure proper performance. The pump starts when there is
sufficient solar radiation available to heat the solar collector.
It shuts off later in the day when the available solar energy
diminishes. As in the previous systems, a thermally operated
valve provides freeze protection.
Common appliance timers also
may control solar system operation. These timers must incorporate
battery backup in the event of power failures.
The timer is set
to operate during a period of the day when solar radiation is
available to heat the potable water. In order to avoid loss of
energy from the tank during overcast days, the collector feed and
return lines are both connected at the bottom of the storage tank
with a special valve. During normal operation, natural stratification
allows the warmer water to rise to the top of the tank.
Figure 2. Direct system with photovoltaic-powered pump
Indirect Pumped System
This system design is common in northern
climates, where freezing weather occurs more frequently. An antifreeze
solution circulates through the collector, and a heat exchanger
transfers the heat from the antifreeze solution to the tank water.
When toxic heat exchange fluids are used, a double-walled exchanger
is required. Generally, if the heat exchanger is installed in
the storage tank, it should be in the lower half of the tank.
The system illustrated in Figure 3 is an
example of this system type. Here a heat transfer solution is pumped
through the collector in a closed loop. The loop includes the collector,
connecting piping, the pump, an expansion tank and a heat exchanger.
A heat exchanger coil in the lower half of the storage tank transfers
heat from the heat transfer solution to the potable water in the
solar storage tank. An alternative of this design is to wrap the
heat exchanger around the tank. This keeps it from contact with
The brain of the system is a differential controller.
In conjunction with collector and tank temperature sensors, the
controller determines when the pump should be activated to direct
the heat transfer fluid through the collector.
The fluid used in
this system is a mixture of distilled water and antifreeze similar
to that used in automobiles. This type of fluid freezes only at
extremely low temperatures so the system is protected from damage
caused by severe cold.
Figure 3. Indirect pumped system using antifreeze solution
Drain Back System
A fail-safe method of ensuring that collectors and collector
loop piping never freeze is to remove all water from the collectors
and piping when the system is not collecting heat. This is a
major feature of the drain back system illustrated in Figure
4. Freeze protection
is provided when the system is in the drain mode. Water in the
collectors and exposed piping drains into the insulated drain-back
reservoir tank each time the pump shuts off. A slight tilt of
the collectors is required in order to allow complete drainage.
A sight glass attached to the drain-back reservoir tank shows
when the reservoir tank is full and the collector has been drained.
In this particular system, distilled water is recommended to
be used as the collector loop fluid- transfer solution. Using
distilled water increases the heat transfer characteristics and
prevents possible mineral buildup of the transfer solution.
the sun shines again, the pump is activated by a differential
controller. Water is pumped from the reservoir to the collectors,
allowing heat to be collected. The water stored in the reservoir
tank circulates in a closed loop through the collectors and a heat
exchanger at the bottom of the solar tank.
The heat exchanger transfers
heat from the collector loop fluid to the potable water in the
Figure 4. Indirect pumped system using distilled water
Integral Collector Storage (ICS) System
In the integral collector storage solar system shown
in Figure 5, the hot water storage
system is the collector. Cold water flows progressively through
the collector where it is heated by the sun. Hot water is drawn
from the top, which is the hottest, and replacement water flows
into the bottom. This system is simple because pumps and controllers
are not required. On demand, cold water from the house flows into
the collector and hot water from the collector flows to a standard
hot water auxiliary tank within the house.
A flush-type freeze
protection valve is installed in the top plumbing near the collector.
As temperatures near freezing, this valve opens to allow relatively
warm water to flow through the collector to prevent freezing. In
south Florida and certain areas of central Florida, the thermal
mass of the large water volume within the ICS collector provides
a means of freeze protection.
Figure 5. Integral collector storage system
A typical thermosiphon system is indicated in Figure
6. As the sun shines on the collector, the water inside
the collector flow-tubes is heated. As it heats, this water
expands slightly and becomes lighter than the cold water in the
solar storage tank mounted above the collector. Gravity then
pulls heavier, cold water down from the tank and into the collector
inlet. The cold water pushes the heated water through the collector
outlet and into the top of the tank, thus heating the water in
A thermosiphon system requires neither pump nor controller.
Cold water from the city water line flows directly to the tank
on the roof. Solar heated water flows from the rooftop tank to
the auxiliary tank installed at ground level whenever water is
used within the residence.
This system features a thermally operated
valve that protects the collector from freezing. It also includes
isolation valves, which allow the solar system to be manually drained
in case of freezing conditions, or to be bypassed completely.
Figure 6. Thermosiphon system
Source: Florida Solar Energy Center