Active Solar Water Heater Systems

Contents

0. Introduction to Active Solar Water Heaters

1. Explanation of Heat Transfer

2. How the System Works

3. Efficiency & SRCC Ratings

4. Vacuum Tubes vs Flat Panel Collectors

5. Installation

6. Calculating Heat Gain / Insolation

7. Types of Solar Water Heat Storage Tanks

8. Adding a Solar Collector to an Existing Water Heater Installation

9. Overheat protection

10. Recommended Accessories

11. The Savings

12. Solar Water Heater Pricing

Introduction to Active Solar Water Heaters

Active solar water heater systems are the most popular type of solar water heaters in North America. They utilize a pump to transfer heat from the collectors to the water to be used, which is stored in a remote location away from the collector. The best tanks to use in these active systems have built-in copper heat exchanger coils so that the heat can be transferred from the working fluid which passes through the collectors and into the water which is in the tank.

One advantage of active solar heaters is that they can store the hot water anywhere, even far away from the actual collectors which receive the sunlight. This allows for strategic placing of the tank close to where the water will be used so that it will not take long for hot water flow.

Since the placement of the tank is normally indoors, heat loss is reduced during winter months due to less temperature differential of the water in the tank and the ambient air outside of the tank. Passive solar water heaters with attached tanks will lose a lot more heat to the ambient air in the winter, when there is less sun to collect energy from, since the tank is kept outside.

The tanks used to store the hot water may also have backup heating systems in case of cloudy days where there is not enough heat to meet the demands of the user. This element is strategically placed in the upper part of the tank so that only some of the water is heated to conserve on energy but still ensure there is enough hot water for temporary use.

Another advantage of the active solar water heater is winterization. Since the working fluid is an intermediate fluid, a non-toxic inhibited propylene glycol can be used to prevent freezing. Although there is less hours of sunlight throughout the days during the cold winters in the north, there is much heat to be collected with a solar water heater.

Explanation of Heat Transfer

There are 3 forms of heat transfer.

1. Conduction
When two objects come into contact, heat transfers between the two surfaces. The heat transfer rate 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 heat pipes to rapidly transmit heat from the tubes to the manifold. The speed of the heat transfer is aided by use of evaporation and condensation of an antifreeze fluid which is inside of the heat pipe. After heat has transferred to the manifold, a working fluid circulates through the manifold to collect the heat from the copper piping inside of the manifold.

The vacuum tubes are very well insulated from heat loss by use of vacuum. They are comprised of 2 tubes, an inner and an outer where a vacuum is applied in between them. While they do not have a full vacuum, they are very close to it with a pressure rating of only 5 x 10^-3 Pa. Conduction requires matter to transfer through. In a pure vacuum, absolutely no conductive heat transfer can occur. In between the tubes there is very little air, so conduction of heat through it to the ambient air outside of the tube is negligible to the efficiency of the solar water heater system.

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 rapid heat transfer to the working fluid. Convection tends to transfer heat much faster than convection, but it also requires matter to transfer heat, so the vacuum in the tubes prevents convective heat transfer to the ambient air.

3. Radiation
Radiation occurs all the time with any object. Heat can travel through a void space through radiation. Radiation, however, transfers at a very low rate compared to conduction and convection. While the tubes do lose some of the heat through radiation, the amount of heat loss is negligible to the solar system since radiation transfers at such a slow rate.

How the System Works

Radiation from the sun beams through the outer glass tubes of the solar collectors. The sunlight then gets trapped into the inner glass tube, which is dark to attract the light and on the inside has a copper lining to collect the heat.

Light which reflects off the dark portion is also trapped. Sunlight easily passes through the clear glass, but as soon as it bounces off of something that isn't clear, it turns into infrared energy / heat. This heat is also trapped into the tubes since heat cannot easily pass through the vacuum (no conduction or convection), and is forced to conduct into the copper tubes in the center.

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. Their typical life expectancy is about 15 years or more, with only a small amount of efficiency loss after about 7 or more years as the age of the tubes takes its toll on the thermal arousing layers.

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.

Efficiency of Solar Water Heaters & SRCC Ratings

SRCC stands for Solar Rating & Certification Corporation. They specialize in rating solar water collectors for their overall efficiency based on different working temperatures and reporting their physical characteristics.

Solar vacuum tubes have a peak solar light absorption efficiency of about 60~80%. While they are extremely efficient on their own, there are many losses to consider before finding the actual amount of heat transferred to water in the system. The amount of insulation on the piping, age of the tubes, insulation in the manifold, working temperature of the collector fluid and ambient temperatures all play a huge role in how efficient the overall system will be.

There are several ways to report the efficiency of a collector. SRCC OG-100 ratings report their efficiency based on gross area. Because the gross area includes the manifold, frame, spaces between the tubes and any other part which does not actually absorb sunlight, this value is only useful when comparing different collectors for the sake of saving installation space.

If comparing the efficiency of different collectors, it is much more useful to compare efficiencies based on aperture area of the collector, which is the part of the collector which actually absorbs the sunlight. To do this from an SRCC rating, take the gross area of the collector, divide it by the aperture area and then multiply by the efficiency rating calculated by SRCC. In the case of our 30 tube collector operating in the A category, with an incident angle of 0 (same as your latitude) this would be be an efficiency of 4.541 / 2.835 * 0.318 = 50.9%. With this efficiency rating based on aperture area, the collector can now be accurately compared to other types of panels such as flat panel collectors.

In the SRCC test, there are

The amount of heat a solar collector can produce in a day depends on the insolation. Insolation can be found online through maps and charts based on location. it varies from the time of year, with less in the winter, more in the summer. When sizing a collector for a heating application, insolation will need to be taken into account to figure out how much heat can be obtained per day. Insolation used in efficiency equations can be used in a mean average throughout the year or for specific time periods. If a system needs to be optimized for the winter (heating a home) the average insolation over the winter should be used. If pool heating is the goal, the summer average insolation can be used. If the application is for strictly water heating, the winter or annual average insolation can be used.

Insolation is given in units of kWh/m² per day (SI) or Btu/ft² per day (IP). This value should be multiplied by the efficiency equation found in the SRCC test to find the amount of heat that will be obtained per day. As an example, with an average insolation of 2000 Btu/ft² per day, with an incident angle of 0 and in the C category (warm solar water heating) our 30 tube collectors produce 9.1 kWh/day or 31,000 Btu/day.

This number can be used to calculate the amount of water heated per day. Water takes 4.186 KJ per Liter to heat it by one degree celsius. So for 9.1 kWh/day, which is 32,760 KJ/day (9.1 kW*60min*60sec), 7826 liters of water (32,760KJ/4.186KJ) can be heated by 1 degree celsius per day. This also means that a 300 liter tank can be heated 26 degrees celsius per day (7826 liters/300 liters).

The 9.1kWh/day was found through the C category, which is rated for a 20°C temperature differential, so if the outside ambient air is 20°C, then the water in the tank would be 40°C, and at the end of the day, would be heated to 66°C in a 300 liter tank. This is, of course, just a rough estimate as the efficiency will go down as the water gets hotter or if the ambient air gets colder, both which are variables that are ever-changing in a solar water heating application. Whenever the water is cold (city water temperature) the collector will be very efficient. As the water goes from warm to hot, the efficiency will drop according to the efficiency equation in the SRCC. Formulas or spreadsheets can be made to calculate a more accurate water temperature at the end of the day if one so desires to.

Vacuum Tubes vs Flat Panels

Vacuum tube collectors are a fairly new technology which are much more efficient in winter applications than their older counterparts, the flat panel collector. The reason primarily for this is the vacuum used to prevent heat loss in the tubes. Flat panel collectors are also one piece units. They are inconvenient to install and maintain. If one part of the collector fails or is broken from a foreign object, the entire collector is ruined. In the case of a vacuum tube collector, several tubes can be broken and the collector will still work. The tubes can be replaced at any time when it is convenient. Also, if the system is aging and losing efficiency, parts can be replaced instead of the entire unit.

Aperture area of a collector is most important when comparing different collectors for their efficiencies. This is crucial when comparing a vacuum tube collector against a flat panel collector. Flat panel collectors are nearly all aperture area while vacuum tube collectors tend to have a much higher gross area to aperture area ratio. Because of this, if efficiencies of gross area are compared, a flat panel collector will seem much better than the vacuum tube. However, if compared by aperture area, vacuum tubes are likely to surpass flat panel in efficiency. So in order to compare them effectively, efficiencies of aperture area should always be compared.

Solar water collectors are most efficient when they are faced perpendicular to the sun. This is approximately the same angle as your latitude. Flat panel collectors rapidly lose efficiency as the sunlight deviates from this ideal condition since they are flat and without a tracking device, cannot face at that ideal angle throughout the day.

Vacuum tube collectors have a 360° degree absorber surface. These tubes passively track the sun throughout the day. This is a key advantage over flat panel collectors which are only most efficient with maximum sun exposure (midday). While in the summer flat panels may show a slightly higher efficiency rating when in direct sunlight, vacuum tube collectors will trump their efficiency by a great amount when the sun is offset from an ideal perpendicular location, such as in the morning or the evening. Therefore, it is not wise to simply compare a flat panel collector efficiency to a vacuum tube collector efficiency simply by its maximum energy output since vacuum tubes have a much longer range for maximum energy output.

Installation

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.

Types of Solar Water Heater Storage Tanks

Stainless Steel

Stainless steel water storage tanks are the best for solar water heater applications. They can withstand the high pressure and excessive stresses put on them by constant heating and cooling throughout the day from the solar water heater. Stainless steel offers excellent corrosion resistance in various water conditions. The only week point of a stainless steel tank is at the seams of the welds. In order to protect the welds, we utilize magnesium anode rods which will corrode first before the stainless steel or any points on the welds can corrode. If these rods are replaced on time, the tank can easily last for a very long time, very possibly a lifetime. Stainless steel tanks are clearly superior over Vitreous Enamel tanks, which are more cheaply produced and can last a long time if maintained properly, but are risky to use in solar applications.

Vitreous Enamel Tanks

VE water tanks are made of carbons steel and utilize a glass lining to protect the metal against corrosion when exposed to water or air. Typical home water heaters are VE style tanks and are generally preferred as they are cheaper than stainless steel. However, VE will fail when a tank reaches temperatures of 70°C or more due to excessive expansion and sticking to the steel. The VE will crack, causing the carbon steel tank to become exposed and susceptible to corrosion. Since such high temperatures are common in solar applications, VE tanks need to be specially designed for solar applications. Usually the glass lining is made to be much thicker so that cracking is less likely under the extreme temperature variations.

Adding to Existing Installations

With an existing tank

Home owners who have recently installed a new water heater tank may not want to replace the tank when installing a solar water heater system. Although the tank is new, one thing to consider is that most normal water heaters are the vitreous enamel (glass lined) type. Although this cheaper style tank is generally just fine against corrosion, it does not handle the high temperatures experienced from solar water heaters. When temperatures reach above 70°C in these tanks, there is a risk of cracking of the glass lining due to excessive expansion and sticking to the steel, thus potentially exposing the steel to the water and leading to unwanted corrosion. These types of tanks which are made for solar water heating do use a thicker layer of lining than typical water heater tanks, but in the end, stainless steel tanks are the best way to go in solar applications.

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 once 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 as an antifreeze in the working fluid, it will become rapidly acidic when above 280°F. The good news is only a small part of the working fluid is exposed to these high temperatures. The vapor inside of the collector pushes the fluid out of the collector and most of it into the expansion case. For short periods of time, this is not a problem, as propylene glycol with corrosion inhibitors helps 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. One 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 through the T/P valve when the temperature gets too high. The collectors can also be covered by a reflective sheet in the summer months so they take in less heat.

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. The solenoid valve can be a combination of 2 of the2-way valves, one normally open, and one normally closed, which are activated at the same time to completely redirect flow, or a high flow 3-way solenoid valve.

Another way to control passing of hot fluid through the radiant heat dump is to use a thermostatic mixing valve. Connect the hot inlet port to the normal loop of the solar system feeding from the collector. Connect the cold inlet to the solar loop portion after the radiator. Connect the outlet of the mixing valve to return to the tank/pumping station. Set the temperature to the desired maximum temperature of the loop and as the fluid starts to get over that maximum temperature, the hot port will start to close and the cold port open so that it forces fluid to pass through the radiator so that the outlet will be no more than the set temperature. This is a great mechanical way to control overheating with use of a heat dump.

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 will help preserve the working fluid from becoming rapidly acidic, the heat pipes also do contain antifreeze inside of them, which is best kept cooler so that it will be less likely to prematurely corrode the copper heat pipes. 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, temperatures can vary from the backup temperature setting all the way to the maximum tank temperature setting. Because of this, a thermostatic mixing valve is a must for solar water systems. It will help protect you from scalding at the faucet, and also protect any PVC or CPVC piping which cannot handle near boiling temperatures. Use a thermostatic mixing valve at the outlet of the solar tank to automatically mix cold water into the hot water to the desired set temperature.

Savings

Water heating accounts for at least 25% of energy consumption in a home. Hot water is needed for showers, hand washing, dishwashing, laundry and many other various applications. Gas is already expensive to heat water with and electrical heating is not much cheaper, never mind 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.

Continue to view available solar water heater systems and pricing.