Solar Hot Water

Case Study - 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 28: 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 cold climates, the need for 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 Results

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 3.0% decrease in the overall whole house energy consumption. Part of the reason for the small savings is the relatively high efficiency of the tankless heating unit it replaces.

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