Designs That Work
Hot-Humid Climate
Advanced Technologies - Solar Hot Water
The incorporation of domestic solar hot water system into residential
homes has become increasingly popular over the last several years. The
basic concept of all solar hot water systems is to use the sun’s energy to
heat or preheat water, thereby reducing the gas or electric requirements
to produce hot water.
In general all solar hot water systems have a solar collector (to
collect the sun’s energy), and a storage tank (to store the hot water).
From this however, the systems can be separated into two different
categories, active and passive systems.
Active systems rely on pumps and valves to circulate the water or heat
exchange fluid through the solar collector, while passive systems rely on
the natural tendency of water to rise when heated, and thereby circulate
through the system.
Figure 31: Schematic of a Closed Loop Solar Hot Water System
While active systems are slightly more complicated than passive
systems, they can be more flexible in terms of the placement of the
components since the location of the storage tank is not dependent on the
physics of hot water buoyancy. On the other hand, passive systems, because
of the lack of pumps have been argued to be more durable and less prone to
problems.
Active Systems
There are three main types of active systems, direct, indirect, and
drain back.
With direct systems, the domestic potable water is circulated directly
through the solar collector. The pump circulates the water from the
storage tank through the solar collector when the temperature of the solar
collector is greater than that of the tank. Direct systems are generally
not recommended for climates where the exterior temperature drops below
freezing or for areas that have hard or acidic water.
For areas where freeze protection of the system is important, the
recommended systems would either be an indirect (closed loop) or drain
back system. The indirect (closed loop) systems use a propylene glycol
heat exchange fluid in the solar collector. The low freezing temperature
of the propylene glycol provides the freeze protection for the system
allowing the solar systems to be used in climates prone to longer freezing
times. These indirect systems require a check valve to prevent reverse
thermosiphoning at night, since the hot water in the tank could convect
heat back up to the typically roof mounted solar panels.
The drain back system uses water as the heat exchange fluid. In order
to provide for freeze protection, the pump shuts off when the temperature
of the collector cools down below that of the tank, and the water in the
system “drains back” into storage reservoirs. The panel then fills with
air protecting the system from freezing when the pump is turned off.
For both indirect and drain back systems, the solar collection loop is
run to a heat exchange coil around a water storage tank. In that way, the
systems are decoupled from the potable water delivered to the house.
Passive Systems
There are generally two types of passive systems; thermo-siphon, and
integral collector storage.
A thermo-siphon system uses the tendency of water to rise as it is
heated. In this system a storage tank is installed at elevation above the
collector. As the water is heated, it becomes lighter, and naturally flows
up and into the top of the storage tank. The cooler water from bottom of
the tank flows down pipes to the bottom of the collector, creating the
circulation through the system. As the temperature in the panel drops
below the temperature of the storage tank, the circulation through the
system stops as well. This prevents the cooler night time temperatures
from removing heat from the system.
Thermo-siphon systems can also be designed with a closed loop and heat
exchange fluid as well, in areas where freeze protection is required.
In the integral collector storage system, the storage tank is
integrated into the solar collector. The cold water supply is connected
directly to the collector. As water enters into the panel it is heated up
by the sun. However, unlike other systems, the water remains in the panel
until there is a call for hot water, and then the water is drawn directly
from the panel to fulfill the demand. Since the hot water is stored in the
panel, integrated systems require larger storage tubes in the collector
(to increase collection ability) than a normal direct system, which also
helps prevent freezing. This is likely the simplest solar hot water system
available.
Design Considerations
The solar collectors should be placed on the South side of the building
with the optimum tilt for the collector to be set to the azimuth angle for
the location of the house. This is to provide the best year round
performance of the system.
Due to the potential for high temperature water leaving the solar hot
water system, a mixing valve must be installed on all systems to regulate
the water temperature delivered to the house, and prevent any concerns
about scalding. In addition, it is generally required to install some
means of providing back up heat with any solar hot water systems to ensure
that hot water demands can be met all year round. The simplest way to
provide the back up heat is with a small electric heating coil inside the
storage tank. Alternatively, instantaneous water heaters can also be used.
If instantaneous water heaters are used for a back up, they must be
designed to handle the potentially elevated water temperatures from the
solar panel.
Energy Model
The system used in the energy model is based on a closed loop glycol
system with a SunEarth Empire EC40 solar collector plate with an 80 gallon
Rheem Solaraide HE (heat exchange) tank. The collector was oriented to the
South and the angle was set to the angle of the roof slope in order to
approximate the most realistic installation of the panel on the roof. The
resultant energy savings was a 11.5% decrease in the overall whole house
energy consumption. |
