Active Solar Water Heater Systems

Explanation of Heat Transfer

1. Conduction
when two objects come into contact, heat transfers between the two surfaces. This is based on the type of materials and contact surface area. copper is without question, the fastest heat conducting material. These solar water heaters use copper tubing to transmit heat from the tubes to the water manifold, which is then collected by the water and then pumped to your water tank.

2. Convection
Convective heat transfer is caused by the flow of a fluid over an object. there is convection caused when the water is pumped through the solar collectors, enabling fast heat transfer and low pumping time. what about the wind blowing over the solar collector? No worry. These are vacuum tubes. The convection must occur directly on the object to cause heat transfer. Wind cannot take any heat out of the system since the vacuum tubes have a void between the copper tubing and the outside glass.

3. Radiation
Radiation occurs all the time with any object. Heat can travel through space through radiation. There's no stopping it. The good news is that the radiation coming out of the solar collectors is extremely minimal, and the radiation coming in from the sun is extremely high due to the black shading of the vacuum tubes and aluminum inserts.

The heat then collects into the center copper tube with help from aluminum inserts. The heat travels up the copper tube and into the manifold through the end bulb (known as the condenser) at the top. This copper tube has water inside of it which helps transfer the heat through evaporation/condensing. Don't worry, this water is under pressure and will not freeze! The heat pipe, which is made of aluminum and is within the manifold is heated by the condenser. Water passes through this manifold, and through the heat pipe. Heat then transfers to the water through conduction and convection. There is very little heat loss since the manifold is insulated well with polyurethane foam and the glass tube is secured against the manifold with a gasket.

Generally, these solar collectors are used in a closed system. They can be used year-round if a water/antifreeze mixture is used. To heat your potable water, you just simply need to run the hot fluid from the solar collectors through a heat exchanger to your water tank, or a special water tank which has a heat exchanger built into it. This is the most efficient way to acquire free, efficient heat.

This is a more efficient system than the more crude systems which actually fill the tubes with water. Not filling the tubes with water also allows for breakage allowance. If one tube were to break, the rest of the system can still operate without the tube until it is replaced.

These solar tubes are made to last. they usually last an easy 10 to 20 years and pay for themselves within 2 years, depending on the cost of your water heating and even building heating needs.

Advantages of the Separated Solar Heater System

Separated systems prevent freezing by containing no water in the tubes. The system uses an efficient copper heat pipe technology to transfer heat from the tubes to the fluid source. A glycol mixture can be used to allow use through all seasons without freezing problems.

Since the system is automated, there's no need to worry about whether you water will be hot. The solar heater will heat your hot water supply and if there isn't enough sun for a few days or you take way too many hot showers, your normal backup heating (electric or gas) can keep your water hot for you.

Stagnation temperature is about 200°C. This is the absolute maximum temperature you can expect the solar heater to reach in case of pump failure and the system stops working. Reaching this temperature will not damage the system.

The Solar Heater

Putting the solar heater system together is quite simple and anyone can do it within about a few hours time. The vacuum tubes come pre-assembled with heat pipes and aluminum fins already inside of them. To install, simply put the base of the frame together, connect the manifold and bottom support, and plug the vacuum tubes/heat pipes into the manifold using the support feet to lock the tubes into place. if this is for a flat roof / ground installation, a little extra assembly needs to be made for the inclined frame. All bolts and nuts are provided and all that is needed for assembly is a wrench or two and at least one pair of hands.

The Plumbing

Although this is a simple installation where pipes only need to be installed to and from the solar heater and to the water tank, all local and other applicable building codes and laws must be obeyed. many do-it-yourselfers can easily plumb this system together but laws may require a certified technician to do so, depending on the law.

Only metal piping and hose able to withstand at least 200°C should be used for the closed-loop system containing the propylene glycol. When the water in the system is not used often, the storage tank may reach maximum temperature and the solar pump will no longer be run until the tank's temperature drops below the maximum setting again. During this time, the solar heater is likely to achieve stagnation temperature, and during the next pump cycle, will delivery very hot fluid to the tank. Metal pipe and high temp hoses should be used incase this scenario occurs. Even if the water storage tank never reaches maximum temperature settings, it is very good to have the correct plumbing in the case of sudden power failure. If the pump cannot operate for even an hour due to some unforeseen event, when it turns back on it could deliver extremely hot fluid through the piping and easily destroy anything which is not resistant to stagnation temperature.

It is common in most applications to use copper piping. However, flexible stainless steel tubing has many advantages over copper. Stainless steel has higher corrosion resistance than copper. The flexible tubing also eliminates the need for the use of elbows in tight turns, where as copper will fracture even if the flexible copper is used. Copper also conducts heat about 20 times as fast as stainless steel, so there will be a lot more heat loss in the pipes when copper is used. Not only will less heat be lost with the stainless steel, but it will also help protect the insulation. Sometimes the fluid gets so hot that the copper will conduct heat too quickly and burn the insulation.This is never a problem with stainless steel.

Rubber type pipe insulation should be used on all piping to preserve heat and protect the building structure. Polyurethane insulation will melt under the extreme heat, so it should not be used. Always use rubber insulation or something else of high temperature resistance. On the outside piping which is exposed to weather and the sun, aluminum wrapping tape should be used to protect the insulation from UV rays and the weather. Left in the open, insulation can deteriorate in only a year or so. If it is properly wrapped, it will last the lifetime of the system.

Calculating Heat Gain / Insolation

More on Sizing the Solar Heaters / Insolation

Heat output from a solar heater depends on many factors such as sun output of the area, time of the year, and weather. The easiest way to estimate the likely potential of a solar heater's output is to use insolation map data. Insolation is the amount of sunlight received on the ground by the sun. Insolation maps can show the average, max and min sun output based on real data collected over the past few decades and can also be displayed for time periods such as winter, summer and averages across the year.

Generally speaking, most parts of the united states average from about 6 to 6.5 KW-hour/m²/day. The average efficiency for a solar heater system is at least 55% for the summertime. This means that for every square meter of absorption area on a solar heater, 6 to 6.5 KW-hours worth of energy times 0.55 is absorbed by a solar collector. When using the value of 6, this comes to at least 3.3 KW-hours worth of energy for 1 square meter.

How to use the insolation calculations

the specific energy of water is 4.314 kJ/(kg*K).1 kg of water is equal to 1 liter of water. For your reference, 1 K (Kelvin) is the same as 1 degree in celsius. A KW is 1 KJ/s. So 1 KW-hour is 1 KJ worth of energy delivered for an hour, or 3600 seconds. This can also be seen as 3.6 KJ worth of energy.

Now let's say you have a 100 liter tank of water and you are using a 1 square meter solar heater at 55% efficiency with the 3.3 KW-hours worth of energy calculated above. The 3.3 KW-hours converts to (3.3)*3600s = 11,880 kJ. Now since it takes 4.314 kJ per liter per deg celsius to heat water, it will take 100 times that to heat 100 liters of water by a degree. So the amount of temperature rise from the heat is 11,880/(4.314*100) = 27.53°C for the 100 liter tank.

With an existing tank

For home water heating installations with existing water storage tanks (water heaters), it is possible although more complicated to install a solar water heater to the existing tank. Since the existing tank does not have a built-in heat exchanger for the solar application, a plate heat exchanger must be used instead. This requires purchase of an additional water pump and a plate heat exchanger. The solar controller can be used to operate both pumps. The solar pump will circulate the propylene glycol mix from the solar heater down to the plate heat exchanger while the 2nd pump will circulate water from the existing water tank to the other ports on the plate heat exchanger. The water flowing through the heat exchanger will receive heat from the propylene glycol mix through the plates, and then be dumped back into the storage tank.

With an on-demand water heating system.

It is quite easy to add a solar water heating system to a currently installed on-demand water heating system. The basic concept is to feed hot water into the currently existing on-demand system and not bother with backup heating in the solar system itself. When heat is lacking from solar, the on-demand system kicks in. Users will still need all of the basic components which are needed for the solar system: solar heater, tank, working station and propylene glycol, but since no backup heating is used in the tank itself, the operation of the system tends to be a little less expensive than using the standard electrical or gas backup heating within the tank itself.

Overheat Protection

Active solar water systems are closed loop. As the temperature of the working fluid increases, so will the pressure. An expansion vessel is used to relieve this added pressure. Should the system shut down the pump in tank overheat protection mode, this expansion vessel is essential for preventing too much pressure build up in the system. As fluid evaporates in the manifold from the excess heat, it pushes the remaining fluid in the pipes down into the expansion vessel. Once all of the fluid has been pushed into the expansion vessel, the system will remain stagnant until the collector cools down and then resume normal operation once the tank temperature protection mode is no longer in effect. Usually once an active system shuts down, it will not resume collecting of heat until the next day.

There are two problems with allowing the working fluid to boil off. The first problem is the obvious, that is, that no more heat can be collected during the day that the stagnation temperature is achieved. Because the pressure is too high within the collector, the pump cannot circulate the fluid. Also, if it could force it, there is a chance of damaging equipment due to the extreme temperatures. The other problem is that if propylene glycol is used in the working fluid, it will become rapidly acidic when above 280°F.

The good news is only part of the working fluid that is exposed to these high temperatures is the vapors in the collector and a few feet of fluid in the pipes to the collector, For short periods of time, this is not a problem, as propylene glycol with corrosion inhibiters will help prevent corrosion to the piping. But if a system is oversized in the summer and overheats everyday, this can become a potential issue without a regular change of the working fluid. Typically the fluid should be changed every 3-5 years. if overheating is common, it may need to be changed every 1-2 years. The use of flexible stainless steel tubing is a good way to counter this. Copper will corrode much more quickly than the stainless steel will.

There are a few counter measures which can be taken against the increase of acidity of propylene glycol due to overheating. The simplest solution is to change the working fluid out with regular water for the summer months when overheating is to be expected, or to dump the water in the tank when the temperature gets too high. Another solution is to use the heat dump function of the controller, where a solenoid valve is activated to allow circulation of the working fluid through a radiator outside, which will dump the excess heat back into the ambient air. Another simple solution is to cover the tubes with something during summer months, which may be more of hassle than it is worth unless the system was oversized on purpose to make sure there was enough heat in the winter (when dumping heat to the home as well as the water tank). Pools are great way to dump excess heat in the summer.

Keep in mind that the ability to dump heat is good for 2 reasons. One is that the system will never become stagnant, and if you begin to use your hot water it can continue heating when it is needed. The 2nd reason is that while you may be able t keep the working fluid from becoming rapidly acidic, the heat pipes also do contain any antifreeze inside of them, which will likely corrode the copper heat pipes faster than with normal operation. This is not really too much of a problem, as the heat pipes have been designed with occasional overheating in mind, but it is still preferable to avoid overheating to preserve the life expectancy of the heat pipes as long as possible.

Recommended Accessories

The sun can be very bright on some days and not out at all on other days. Because of this,

Savings

Water heating accounts for at least 25% of energy consumption in a home. Hot water is needed for showers, handwashing, dishwashing, laundry and many other various applications. Gas is already expensive to heat water with and electrical heating is not much cheaper, nevermind the lack of power given by an electric water heater. When a solar water heater is installed, a home can enjoy the benefits of free hot water with minimal expense to maintain. The Electrical components of the system, when actively running a pump, use about only 40 watts, and with no pump running, the electrical usage for the display is negligible. Even if the pump ran non-stop for 8 hours a day, the cost to run it would amount to only about 3 cents a day.

Most homes will find that a solar water heater can pay for itself in about 1-2 years, depending on the location and costs associated with heating water the traditional way.