SOLAR HOT WATER HEATING

information provided by Home Power Magazine, article written by Ken Olson (edited).

Closed Loop Antifreeze Systems

The major parts of a closed loop, antifreeze type system include solar collectors, circulating pumps, a differential control with sensors, heat exchangers, and storage tank. Lesser but essential parts include an expansion tank, pressure relief valve, check valve, drain/fill assembly, and pressure and temperature gauges.
Flat plate solar thermal panels are connected in parallel, and usually installed on a roof or mounted on a ground structure. The closed loop components are fastened to the wall near the solar storage tank. Finally, the heat exchanger, solar storage tank, and collectors are connected to the closed loop components. Let's take a closer look at the function of each of these components.

Collectors

Flat plate collectors are most commonly used for low-temperature applications (up to 140°F; 60°C), such as residential domestic water heating and pool heating. Water passes through parallel copper tubes bonded to a flat copper sheet under glass, all enclosed in a weathertight insulated frame. For higher temperature applications (over 140°F), evacuated tube collectors are more efficient. The design of evacuated tube collectors reduces heat loss caused by convection, radiation, and conduction.

Circulating Pumps

Centrifugal-type circulating pumps are most commonly used in solar hot water systems and hydronic heating applications. Centrifugal circulating pumps are appropriate for their low power consumption, low maintenance, and high reliability. They typically are made with cast iron, bronze, or stainless steel bodies.
For closed loop systems lower cost, cast iron circulating pumps are adequate. For the open loop part of the system, which circulates a replenishing supply of water, a bronze circulating pump is necessary. Stainless steel circulation pumps are used in pool systems and other applications where chemicals are present.
Once you know whether your circulating pump is to operate in a closed loop, open loop, or other particular environment, pump selection will be based on head and flow requirements. Head is the pressure the circulating pump must develop in order to create desired flow through the system. The overall pressure a pump must create is determined by the height the water must be lifted and the frictional resistance of the pipe.
Static head is pressure resulting from the vertical height and corresponding weight of the column of fluid in a system. The higher a pump must lift the fluid against gravity, the greater the static head it must develop. Dynamic head includes the frictional resistance of the fluid flowing through the pipe and fittings in the system. The pressure a pump must develop to overcome dynamic head varies with the size and length of the pipe, number of fittings and bends, and the flow rate and viscosity of the fluid.
Circulating pumps are typically categorized for low, medium, or high head applications. Low head applications have 3 to 10 feet (0.9-3 m) of head; medium head applications, 10 to 20 feet (3-6 m) of head; and high head applications, over 20 feet of head.
A pump curve is supplied for each pump by its manufacturer (see the pump curve diagram above). The flow rate, in gallons per minute is shown across the horizontal axis. The vertical axis shows the pressure as measured in "feet of head." Each pump has its own curve, which shows the volume of flow it will create at any particular head it must overcome. If you refer to the pump curves above you will see that the Grundfos model 26-96 F will push 20 gallons (72 l) per minute (gpm) against a resistance of 10 feet (3.1 m) of head. Ten feet of head is equal to 4.3 psi (10 ÷ 2.31 feet per psi = 4.3 psi).
For the solar closed loop, you can select a low head circulating pump with a cast iron or bronze body. You might consider the Grundfos model UPS 15-42 F to cover a wide margin of error. This model is a three-speed circulating pump that allows you to select the most appropriate speed once installed. A low head bronze pump will suffice for the water loop.
Whether open loop or closed loop; high, medium or low head; your local plumbing supply house probably carries an appropriate pump without special order. Most solar domestic hot water systems can be served by the Taco 008F, Grundfos 15-42 F, or Hartell MD-10IU pumps for the glycol closed loop; and by the Taco 006B, Grundfos 15-18 SU, or Hartell MD-3IU for the open loop domestic hot water.

Controller & Sensors

The controller is the brains of the system. It tells the pump when to turn on and off, based on collector and storage tank temperatures. All of its intelligence is based on determining whether the collector outlet is sufficiently warmer than the bottom of the tank to warrant turning the circulating pump on.
Sensors are located at the collector outlet, and at the bottom of the solar storage tank. These sensors are thermistors that change their resistance with temperature. The differential control compares the resistances of the two sensors. It turns the pump on when the collectors are sufficiently warmer (20°F; 11°C) than the bottom of the solar storage tank to collect useful heat.
The Independent Energy GL-30 is an example of a good differential control. It has an adjustable setting on the order of 5 to 25°F (3-14°C) temperature differential. These controls also have a high limit cut-out that will shut the system down once the tank reaches a predetermined high temperature, adjustable from 110 to 230°F (43-110°C). The GL-30 uses 10 K ohm sensors, which are the standard of the industry today. A 10 K sensor reads 10,000 ohms at 77°F (25°C). Temperature sensors must have the proper resistance to be compatible with a given controller. Sensors are available from the distributors who carry the controller you are using.

Heat Exchanger

The heat exchanger transfers heat from the solar-heated closed loop to the domestic water. Factors that increase heat transfer are:
* Greater surface area
* High thermal conductivity
* Maximum temperature differential between the two fluids
Heat exchangers may be categorized as single wall or double wall, which refers to the number of barriers between the two fluids exchanging heat. Single wall heat exchangers are usually not permitted in potable (drinkable) water systems when a nonpotable heat transfer fluid is used. For example, systems that use glycol should not use a single walled heat exchanger because of the potential for contamination of the potable water.
Double wall heat exchangers are required to ensure that the heat transfer fluid will not contaminate the potable water. The space between the two walls of the heat exchanger is usually vented to permit detection of a leak.
A heat exchanger may be as simple as a copper coil within the storage tank, where single wall heat exchangers are permitted. Solar heated water is circulated through the coil with the help of a circulating pump. Heat is transferred by natural convection to the water within the tank.
External heat exchangers are generally less costly than a custom solar tank that includes an internal heat exchanger. External heat exchangers typically require two circulating pumps-one for the collector loop and another for the water loop. This uses more energy to run the second circulating pump, but it is more effective than an internal heat exchanger that relies solely on natural convection on the water side.
Heat transfer is driven by temperature differential. For this reason, heat exchangers are installed in a counter-flow configuration whereby the two fluids flow in opposite directions through the heat exchanger. This maximizes thermal heat exchange by maintaining the greatest temperature differential between fluids.
Selection of a heat exchanger is based on its capacity to transfer heat (in BTUs per hour) produced by the solar collectors. The manufacturer or distributor of the heat exchanger will specify which model is adequate, depending on the total square footage of collector to be installed. They will also specify a recommended minimum flow rate on the water side of the heat exchanger to achieve an adequate rate of heat transfer.

Check Valve

A check valve permits fluid to flow in one direction only. It prevents heat loss at night by convective flow from the warm storage tank to the cool collectors. Check valves may be of the "swing" type or the "spring" type.
Swing-type check valves should not be installed vertically upside-down, since they can hang open. If you are powering your circulating pump directly from a PV module, you should use the swing-type check valve. Low sun conditions produce lower flow rates, which may not be strong enough to overcome a spring-type check valve. For systems using AC circulating pumps, spring-type check valves are preferred. The spring provides a positive action against thermosiphon flow in either direction.

Expansion Tank

An expansion tank allows for the fluid in the closed loop to expand and contract in the cycle of heating and cooling. Without the expansion tank, the plumbing would easily burst when the fluid is heated.
Diaphragm-type expansion tanks use an internal bladder and pressurized air chamber precharged at 12 to 15 psi. The solar-heated fluid expands in the closed loop against the bladder and pressurized air chamber. As the fluid contracts while cooling, the air chamber maintains pressure in the closed loop.
Diaphragm-type expansion tanks may be installed in any orientation, but if inverted such that the air chamber is above the fluid, it will continue to function even when the bladder eventually fails.
The size of the expansion tank must be able to handle the expansion based on the volume, coefficient of expansion, and range of temperature fluctuation. These factors are considered in the rule-of-thumb recommendations below, which you may use to estimate expansion tank size based on total fluid volume.
The size and number of collectors, and the size and length of piping and fittings determine fluid volume. Use a #15 expansion tank for volumes up to 4.7 gallons (18 l), and a #30 expansion tank for volumes up to 12.5 gallons (47 l). Multiple expansion tanks can be used to increase capacity if necessary. Extrol diaphragm-type expansion tanks are readily found in most plumbing supply houses.

Pressure Relief Valve

Every hydronic heating system must allow for protection against excessively high pressures due to high temperatures. A pressure relief valve of 50 psi is typically adequate to protect closed loop plumbing from excessive pressures.
Temperature/pressure relief valves are not commonly used in the closed loop because high temperatures can be a frequent occurrence. Pressure-only relief valves are most commonly used. Once one of these valves opens, it is wise to replace it, since they often may not reseat, leaving a slow but persistent leak. Pressure relief valves should be fitted with a vent tube to direct vented fluid to a bucket or floor drain.

Gauges & Meters

A pressure gauge will tell you if the closed loop is within an acceptable range of pressure. A typical system pressure is on the order of 12 to 15 psi. A pressure gauge is used as a diagnostic tool to monitor the state of the glycol charge.
Two temperature gauges in the closed loop and one in the water loop are optional indicators of system function. One gauge on each side of the heat exchanger in the collector loop will show the temperature rise across the collectors and the temperature change across the heat exchanger.
A temperature difference of 15 to 20°F (8-11°C) indicates effective operation. One temperature gauge in the water loop between the exit of the heat exchanger and the entry to the storage tank will display the current temperature of solar heated water.
Select a temperature gauge with a range of 0 to 250 or 300°F (-18 to 120 or 150°C). A hot summer day may produce water temperatures exceeding 200°F (93°C), although normal high temperatures are usually around 180°F (82°C).

Antifreeze

The collector loop circulates an antifreeze solution. Propylene glycol is the most common heat transfer fluid. It is a non-toxic substance, and more commonly used as a food additive, though it is not considered a potable fluid. Propylene glycol is usually mixed in a 50:50 solution with demineralized or distilled water. Inhibitors may be added to increase the life of the fluid, which breaks down over time due to overheating. It then forms a sludgy deposit that can clog the collector loop, as well as reduce the solution's effectiveness as an antifreeze.
Ethylene glycol should never be used. It is the common antifreeze used in automobile coolant systems. It is highly toxic, and will cause a great deal of discomfort and death if consumed.
At the time of installation, the collector loop is charged to operating pressure with a positive displacement pressure pump. Positive displacement pumps have the capability of creating sufficient pressure to lift the fluid the full height of the system, and bring the system to operating pressure, typically about 20 psi. Positive displacement pumps also create sufficient suction head to draw the charging fluid from the bucket.
The charging pump is not a fixture of the system. It is a separate piece of service equipment used by the solar contractor. It is connected to the drain/fill assembly in the collector loop, which consists of two boiler drains with a shutoff valve between them.

Solar Storage Tank

Two tanks are more efficient than one. A solar storage tank may be installed in addition to your conventional hot water tank.
For those summer months when you can be satisfied with solar hot water alone, you can install a "bypass valve assembly" between the solar storage tank and the backup water heater. The solar bypass consists of three valves (or two 3-way valves), which allow you to give your conventional water heater a summer vacation and supply the house with solar heated water directly.
If you like that idea, you should add a measure of protection for those days when you just might get water hotter than you can handle. A tempering valve can keep you from getting scalded when your solar heated water is hotter than you normally enjoy from a thermostatically controlled conventional tank. The tempering valve is installed between you and your hot water system. With it, you can set the desired maximum temperature of the water delivered to the tap. Hot water enters one side, cold water, if necessary, enters from the bottom and mixed water goes out to the tap. Contractors who install solar storage tanks to the OSEIA Tank Standard (i.e. high insulation levels on tank and pipes) receive an energy savings bonus for their clients that results in larger savings and tax credits.

For more information and pricing, please contact your local contractor .