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Solar Domestic Hot Water Heating Information, How Basic Systems Work

How a solar water heating system works

Without a solar system, every time you use hot water, cold water enters your hot water heater and the heater turns on. A solar water heating system with solar added to it simply heats the cold water before it enters your water heater. This means that your hot water heater will not have to heat the water as much, and at times not at all. The solar system captures the heat from the sun through the collectors, which are mounted facing the sun. They channel that heat down to the solar storage tank, and then transfer that heat to the cold water before it enters your hot water heater. 

Basic Components of Most Solar Thermal Systems

Solar Collectors

Flat plate solar collectors are the most commonly used. Over the past twenty years they have proven themselves as reliable, tough and efficient. They are available in many different sizes, the most common being 4'x8' and 4'x10'. The collectors are typically the most expensive components of your solar system. Below are a few important items every flat plate collector should have;

1. A warranty
2. Low iron or low-e Tempered glass
3. Selective coating on the absorber plate
4. Insulation
5. Copper manifolds and risers

Heat Exchangers

A heat exchanger simply allows two fluids to transfer heat without mixing the fluids. There are many ways to do this, some very efficient and some not. The most common heat exchanger most people are familiar with is the radiator in their car. The coolant fluid collects heat from the engine and transfers that heat to the air (yes, air can be considered a fluid). The simplest heat exchanger is simply a coil of copper pipe suspended in a storage tank. These heat exchangers are frequently used in drainback solar systems to transfer heat from the solar storage tank to the cold water before it enters the hot water heater.

Storage Tanks

Storage tanks are necessary to contain the heat collected by the solar system. Nearly all solar thermal systems today use water. In the past many different fluids and materials have been tried. Water is the most common choice for many reasons.

• Water is capable of storing more heat per pound than most fluids and materials.
• Water is easily available.
• Water is cheap compared with other alternatives.
• Water is not highly corrosive.

There are basically two kinds of solar storage tanks, pressurized and unpressurized. Your hot water heater is a good example of a pressurized storage tank. Pressurized solar storage tanks are the same as your hot water heater tank. They are usually bigger though. Unpressurized solar storage tanks are typically made to fit your space needs and solar system size. Most commonly they have a stiff structure built of wood or steel and are made waterproof by placing a rubber liner inside the structure. Unpressurized stainless steel tanks are also available, although they are more expensive than the others, they can last longer and do not require a liner. Pressurized tanks are preferable for solar systems that are going to have a year round need for hot water. Such as domestic hot water, and where a person does not want to have to maintain the level in the solar storage tank. With the unpressurized tanks, water will evaporate out of them and should be replaced 1-3 times per year depending on your solar system. Pressurized tanks remain filled all of the time by city or well water pressure. One of the biggest benefits of unpressurized solar storage tanks is that they can be built to match e size that the solar system requires. With pressurized tanks you typically are left to work with what is sold. Unpressurized tanks can also be pre-built and reassembled in a basement, crawlspace or tight corner that would not allow a pressurized tank. All tanks are capable of being ordered with a heat exchanger inside the tank. With the pressurized tanks this tends to run the price up quickly though.

Pumps

Most solar systems use Centrifugal pumps. Some solar pumps can be expensive. The more commonly used a pump is, the cheaper it will be.

There are Two Basic Types of Systems

System Control

A differential temperature controller controls nearly all solar thermal systems. The control works verysimply, by measuring the difference between two sensors. One sensor is located near the bottom of the storage tank and the other sensor is located inside the collectors ( or on the pipe coming from the collectors to the storage tank). When the sensor in the collectors is much hotter than the sensor in the storage tank, the controller turns on the pump, circulating fluid through the panels (and through the heat exchanger if the system has one) and collects the heat in the storage tank. These controllers also have a high limit safety so that you will not overheat the storage tank. Some controllers display the sensor temperatures.

Open Loop

Also known as a drainback, these systems are the best choice if you want a bigger system that will provide seasonal heat. To protect themselves from freezing they drain when not in use. The whole system, including the panels and pipes is plumbed with slope. That way, whenever the system is not running, there is no fluid in the system, all of the fluid has drained back into the storage tank (see schematic 1) or into the drainback acculators (see schematic 2). Because the system needs to be able to drain it can not be pressurized by city water and must be separated by a heat exchanger. There are basically two ways of achieving this. You can set up the system such as in schematic 1, where all of the fluid in the tank runs through the panels and there is one heat exchanger to pre-heat domestic hot water (or more heat exchangers to heat a pool, home, spa....). Shematic 2 shows the second way, a system where a smaller amount of water is circulated through a smaller tank, the drainback accumulator, that runs through a heat exchanger. The heat exchanger then heats up water in the storage tank. This enables the use of a pressurized storage tank.

Closed Loop

These systems are always filled and protect themselves from freezing temperatures by mixing a fluid (propylene glycol) with water 50/50 to reduce the freezing temperature lower than -20F. As shown in schematic 3, closed loop systems use a heat exchanger to separate the fluid from the potable water and a second pump to circulate the water in the storage tank. These systems are typically filled to a pressure of about 25 PSI. Because they are pressurized they can use smaller pumps than the open loop systems. These systems are typically best suited for solar systems that are always used, such as a domestic hot water solar system or a indoor pool. These systems are also simpler to install since you do not have to slope the piping.