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In Depth Solar Water Heating System Information

 Solar Water Heating Systems

There is a lot to know when comes to choosing a solar water heating system!

We have broken it down into several sections to make it easier to follow:)

Updated: to include information on PV hot water systems.

Within the text of the information below we have added information on how the new PV hot water systems compare or address the various issues in each section.

1.) Introduction

2.) The collector issue

3.) Seasonal vs. Year-round

4.) Drainback vs. Antifreeze

5.) Electric vs. PV powered solar DHW pumps

6.) System Sizing

7.) Payback / Economics

(By The Way - don't forget to check our Solar Hot Water FAQ for quick answers to those Frequently Asked Questions!)

1.) Introduction

Solar water heating systems are designed to use the radiant energy of the sun to heat the water used in the home for baths, showers, laundry, etc. Solar water heating is one of the oldest applications of solar energy. Solar water heating was commonly used in Florida and California in the late 1800's. It is now very common in other parts of the world. Japan, Israel, Greece, Australia, Germany, China all have large numbers of solar water heating systems!

Despite the fact that most parts of Canada have ample amounts of sun, solar water heating has not caught on here - yet. This is because the low cost of conventional fuels has made using solar unattractive financially. We also have a climate that's tougher on solar systems than most places (snow, freezing conditions, etc.), so our systems tend to be somewhat more expensive to build and install. However as conventional fuel costs go up (and up and up), solar water heaters will begin to be considered a good investment.

Regulations are catching up to the growing interest in solar water heating. Although there have been CSA standards in place for solar products for almost 30 years in Canada, the interest in having products certified to those standards has only just caused a CSA certification process to be put into place! As of 2009 you can buy CSA certified solar water heating systems.

It is generally accepted that approximately 20 to 25% of the energy used in a home in a year, is used to heat the hot water consumed in the home. That represents a tremendous amount of energy. (For comparison - only 5% of the energy used in an average home is used for lighting - less if compact florescent bulbs are used!)

The solar collectors used in a solar DHW System generate heat, not electricity from the sun, and transfer it to the water used within the home. * Of course this is not the case for PV solar DHW systems - they do use electricity generated from the sun.* There are several different strategies and types of equipment used to do this. Almost all solar water heating systems are designed to perform as pre-heaters for existing conventionally fueled water heaters. "Solar only" systems operate without a backup heater and cannot be relied upon to always provide hot water. This is why "solar only" systems are not common in Ontario, and if used are most often found on cottages used only in the summer.

Some of the main issues of solar system design will be covered in the following sections. An issue that usually comes up first, is the type of solar collector to be used on the solar system....

2.) The collector issue

There are two major styles of solar collectors available: Flat-plate (FPC) and Evacuated-tube (ETC). Flat plate collectors have been around for at least 200 years, evacuated tubes have been on the market for about 25 years.

Manufacturers of ETC claim their collectors out-perform flat plate collectors. Field and independent laboratory tests do not support these claims. In our climate, winter snow can seriously reduce the performance of ETC. Because the ETC tubes do not get warm, they cannot easily melt or shed the snow or frost that builds up on them. Snow or frost on a FPC will melt or slide off quickly on a sunny day, even in the middle of the winter. Solar electric (PV) panels lose snow on their faces faster than ETC but not as fast as FPC. An issue with PV panels is that if they are partially covered with snow their performance is severely reduced. If possible, they should be installed where the snow can be cleared from them.

Flat Plate on Left (clear) -  Vacuum Tubes on Right (not clear)

 

 The Flash movie above illustrates the difference between three different types of solar thermal collector. The vertical scale represents the efficiency of the collector. The horizontal scale represents the temperature difference between the collector and the outside air temperature - divided by the intensity of the solar energy on the collector. As the graph shows, the greater the temperature difference the lower the efficiency of all three types of solar collectors. The pool collectors lose efficiency fastest because they are unglazed - and lose their heat to the air very quickly. The Glazed Flat Plate Collectors lose their efficiency faster than the Evacuated Tube Collectors. What is important to note, is where the collectors actually operate on the efficiency curve when installed in the field.

The pool collectors will typically operate with a small temperature difference - as much as 25 Celsius degrees (45 Fahrenheit degrees) and still deliver heat to the pool at 35% efficiency. Glazed FPC will operate with a larger temperature difference - as much as 55 Celsius degrees (99 degree Fahrenheit) and deliver heat to the solar storage system at 45% efficiency. Until the temperature difference (between the solar collector and the ouside air temperature) reaches 55 Celsius degrees (99 Fahrenheit degrees) the ETC are less efficient than the FPC. In real world terms this means that FPC are operating more efficiently than ETC 95% of the time when heating water for residential systems.

When comparing the performance curves from FPC and ETC, it is very important to know if the ETC curve is based upon the gross-area of the solar collector. If it is not, then it is not a valid curve to compare to a FPC curve. Insist upon seeing a curve based upon gross collector area. ETC manufacturers will use other curves based upon net absorber area or net glazing area, but these curves ignore the spaces between the absorbers or the glazing (tubes) and the space taken up by the header assembly. The performance curves for FPC made in North America are all based upon the gross collector area.

 

 

Evacuated Tube collectors (ETC)

AVAILABILITY Many Canadian & US manufacturers of flat plate collectors. No North American manufacturers of ETC tubes. All are made in Europe, Australia, China and elsewhere. Many new & inexperienced solar companies pushing Chinese made ETC collectors. There are many domestic suppliers of PV panels.
DURABILITY FPC use tempered glass - much tougher than ETC annealed glass tubes. Tempered glass will withstand hail & vandalism much better than annealed glass. Vacuums in ETC will be lost over time - especially in some designs (with metal to glass seals). Site assembly of ETC makes them vulnerable to assembly quality issues. PV panels use tempered glass and have performance warranties of 25 - 30 years - warranting that they wiil still have at least 85% of their original performance.
PERFORMANCE For low temperature applications (30 to 70 degrees Celsius, 86 to 160 degrees Fahrenheit) such as pool heating, domestic water heating and radiant floor space heating, FPC are typically superior in performance to ETC. ETC are superior for high temperature applications (70+ Celsius, 160+ Fahrenheit) such as space heating when using baseboard radiators and industrial process hot water or steam. ETC are also better for absorbtion air-conditioning. PV panels are not affected at all by the temperature of what the system is heating. They gain performance as the ambient air temperature cools below 45 C (112F).
COST ETC from Europe are expensive compared to FPC. ETC from China are usually less expensive than FPC. maintenance is simpler and less expensive. PV panels cost less than half the cost per unit of area of FPC and require no maintenance.
WEIGHT For a given collector area FPC are usually much lighter. PV panels are lighter than FPC.
ADAPTABILITY Most North American FPC are built so they can be drained completely. Most European FPC do not drain easily and many ETC trap a significant amount of fluid in their headers that cannot be drained by gravity. This means they can only be used in closed loop , anti-freeze systems and that servicing fluid in the systems is much more tedious and wasteful. PV water heating systems do not have this concern.

 

3.) Seasonal vs. Year-Round

Solar PV heating systems are not advesely affected by freezing tempertures - so there is no discussion around seasonal performance about them. In fact, PV panels perform significantly better as the get colder - the opposite of both FPC & ETC.

A seasonal solar water heating system (SDHWS) is a system that is intended to be used only when the conditions outside will not cause the collectors or the piping to freeze. In Ontario, this means from the mid-Spring (Approx. May 1) to mid-Fall (Approx Sept. 30). What we consider to be a seasonal design in Ontario, is the most common configuration in the rest of the world - as most places using solar DHW systems don't get freezing temperatures!

Seasonal SDHWS use water in the collectors and must be drained before the onset of hard freezing weather. Within this limitation, they are often the simplest, most efficient and cost effective systems available.

There are a number of different configurations of seasonal SDHWS. The simplest is the black barrel. A black barrel, or a water heater tank with no jacket or insulation - painted black, when left sitting in the sun in a warm climate, can heat up 10 to 15 degrees (C) warmer than the ambient air temperature. To improve on this the barrel or tank can be placed inside an insulated box with either a removable top/side or covered with a transparent glazing such as plastic or glass. The removable section or glazing are used to allow the sun inside the box to heat the tank. This style of SDHWS is called a "Breadbox" solar heater. Breadbox SDHWS are heavy, bulky and slow to heat up due to the large ratio of volume of water to solar collection area they provide. They do work and are easy to build.

A method of improving the performance of breadbox SDHWS is to enlarge the solar collector area of the system. This is main feature of Thermosyphon SDHWS.

In most thermosyphon systems, the solar collector is separate from (but still very close to) the storage water tank. In other words the storage tank is completely insulated and does not collect energy directly form the sun. The term "Thermosyphon" refers to the method of transferring the heat from the solar collector to the storage tank. This process does not require an external power source - it uses the fact that hot water is less dense (lighter) than cold water. Hot water will naturally rise to sit above cold water in the same tank.

Thermosyphon systems can have the tank separate from the collectors (as shown in the illustration below, or attached to the collectors (horizontally) as shown in the following photo.

 

A variation on the breadbox and Thermosyphon designs of SDHW, is the Integrated Storage Collector solar water heater - or ICS. Available commercially, typically for use in the Southern US, this design usually consists of a number of small diameter tanks (often sections of 3" or 4" copper pipe) piped together, painted black and enclosed in a glazed box. (They tend to look like fat flat plate solar collectors:) This design improves the volume to absorber area and can work quite well in warmer climates. They do tend to lose heat quickly overnight.

Another common system design, which is seasonal in Ontario, is called the "Direct" SDHW. In this design, the consumed water is circulated between the solar collectors on the roof and the solar storage tank (usually in the basement). Heat exchangers are not used to separate the water used in the home from the water heated in the collectors. This makes the systems very efficient, simple and relatively low-cost, because they do not require all of the hardware and complexity of an anti-freeze system. These systems have two drawbacks. 1.) the collectors can scale up inside if the water in the home is "hard" or has a high dissolved mineral content. (Using softened water will significantly reduce this problem.) Scaling will reduce the efficiency of the collectors over time. 2.) The systems are vulnerable to freezing. Systems of this design must be installed carefully so the collectors and piping exposed to freezing temperatures (especially in the attic) will drain thoroughly when the system is shut down for the winter. The solar collectors in these systems can freeze very quickly because they contain only a small amount of water. (The large amounts of water in ICS systems makes them slower to freeze.) The piping exposed to freezing temperatures can also freeze, although the collector will usually freeze first. (The pipes are insulated - the front of a collector is not.) To be clear why freezing is a problem - when water freezes in a collector or a pipe - it causes them to burst. When the pipes or collector thaw-out, there will be a flood!! If that pipe is in your attic, it will flood the house!

A variation on the direct system, currently being sold in Ontario, is the evacuated tube, heated-storage-tank solar system. This product is only certified to be used seasonally, however unscrupulous vendors represent them as being suitable for year-round use. In this system evacuated tubes transfer heat directly to the body of a stainless steel storage tank mounted on a lightweight frame above them. Water lines transfer city water under line pressure to and from the solar storage tank which is typically placed at the peak of the roof. During periods of cold weather the water lines will freeze and block the flow of hot water to the house. To prevent this from happening the lines are often traced with electric heating cable to prevent them from freezing. (Note: the cost of the electricity to prevent the lines from freezing can easily be greater than the value of heat delivered by operating the "seasonal" system over the Winter!) There is also the significant chance that frozen lines will rupture, and during a prolonged power outage, so could the storage tank, leading to extensive water damage.

In Ontario, most seasonal systems have been used on cottages. There is no reason they can't be used seasonally on urban homes as well. About 70% of the heat collected by a year-round SDHW system, is captured the "seasonal system" usage period. Solar DHW systems do capture most of their heat during the summer.

4.) Drainback vs. Antifreeze

PV solar water heating systems do not use heat transfer fluids (glycol or water) so all of the issues associated with heat transfer fluid use do not exist for them.

It would be natural to think that because we live in an area that experiences a large number of days with (very) freezing temperatures, that you have to use an anti-freeze solution in a solar water heating system that is used all year. This is definitively not the case! There are a number of ways to use water in the solar loop without having a freezing problem.

One way to use water in the solar collectors all year is with a design that drains the water to the drain whenever the temperature outside is close to freezing and the system is not trying to collect heat (no sun available). This "Draindown" design was used a great deal in the U.S. in the early days of solar (1970's). It relied upon solenoids that would open and drain the water from the solar piping and collectors that were exposed to freezing temperatures. What was discovered was solenoids fail - either they failed to open or failed to close, and systems froze. This approach, although it made a very efficient solar system, has been almost completely abandoned.

The best way to use water in the solar collectors is in a "Drainback" design. The drainback design uses a heat exchanger to separate the water that flows through the collectors from the water that is actually used. There are at least two types of drainback system design. The two most common are the 1.) loadside heat exchanger and 2.) the solar side heat exchanger.

In the loadside heat exchanger design the water circulated through the solar collectors is contained in a large (usually unpressurized) tank and is pumped to the collectors and drains back to the tank. The return pipe is open to the air above the tank and air can flow backwards into the solar collectors and piping whenever the pump stops pushing water up the supply pipe. Heat is transferred to the load (water that is actually consumed) by means of a coil-in-tank heat exchanger (usually). The rate at which heat is delivered to the load is limited to the rate at which the heat exchanger can transfer heat from the storage tank. This will be a problem if the heat exchanger is too small.

A drainback system with a solar side heat exchanger, starts with a much smaller amount of water in a drainback tank. The tank is often sealed and under a low pressure, which means its water can't evapourate, and doesn't need to be topped-up, which is frequently the case with load-side drainback tanks. In a solar side heat exchanger, the water from the sealed drainback tank is pumped into the collectors and this pushes the air in the collectors down into the drainback tank, where it is held until the pump shuts off, and then it rises back into the collectors and piping as the water drains back into the tank. The heat exchanger is in the solar loop and heat from the collectors is transferred to the water stored, under pressure, in the solar water storage tank. In this design the amount of heat delivered to the load is not limited by the heat exchanger.

Antifreeze systems normally use a propylene glycol solution, typically 40% to 50% glycol by volume, that has a freezing point of -20 to -30 degrees Celsius. If temperatures below this are reached, these solutions do not freeze solid until they get much colder, but form a slush that won't split pipes - even if it can't be pumped. Antifreeze systems require additional components in the plumbing to deal with several issues they have. An expansion tank is required to absorb the expansion of the antifreeze as it heat up. A pressure relief valve is required on the antifreeze loop to prevent if from over-pressurizing.

One of the biggest issues antifreeze systems have is overheating in the Summer. If there is no hot water load for several days in the Summer (for example, the occupants have gone away on vacation.) the solar storage tank will "fill up" with heat and the solar system will shut itself off. Unfortunately, the collectors are still sitting in the sun and the antifreeze inside them is not circulating. The antifreeze can literally start to boil and raise the pressure in the solar loop. If the expansion tank cannot accommodate this, the pressure relief valve will open and dump antifreeze from the loop, which may mean there won't be enough left in the loop for the system to function properly once hot water is being used again. Even if the antifreeze isn't dumped, boiling it damages it. If this happens often enough, at high enough temperatures, the antifreeze could become very acidic and start to attack the piping, collectors and heat exchanger. If corrosion inhibitors are used in the antifreeze, they can come out of solution and form a precipitant that can plug pumps, small pipes and heat exchangers. This is why antifreeze solutions need to be checked every 2 years and are typically replaced every 3 to 5 years.

Some antifreeze solutions are toxic - mainly because of the corrosion inhibitors added to them to lengthen their service life. For this reason, some regions require that only special heat exchangers be used - typically double-walled units that can't leak glycol into the heated water. This requirement means that either more expensive or less efficient heat exchangers must be used.

5.) Electric vs. PV powered solar DHW pumps

Solar PV water heating systems do not use pumps - so they have no parasitic energy consumption.

Thermosyphon solar systems don't require pumps and don't consume energy while collecting energy. Conventional solar water heating packages have one or two pumps and a solar controller - these items consume electrical power and effectively reduce the savings from the solar water heating system. This is called parasitic energy loss.

At least one solar domestic water heating package uses a small PV panel to power the solar loop pump and controller logic. There is a natural correlation between the amount of energy available for  water heating and the power delivered to the solar pump. This causes the pump to operate at a higher speed when the sun shines strongest this can improve the efficiency of the solar collectors and the heat exchanger resulting in more heat being collected.

6.) System Sizing

Solar PV water heating systems use simple rule-of-thumb sizing formulae, similar to that used by other types of SDHWS. As PV panels have a lower peak efficiency than both FPC and ETC, they require a larger surface area of panels. At current PV efficiencies of 16% - 18%, to get the equivalent performance to a FPC DHWS, the PV area must be increased by about 2.5 times. This equates to about 7.5 sq. metres of PV panel to 3 sq. metres of FPC. (Enough for a family of 2 people.)

Sizing solar water heating systems for single family residential applications is not that difficult. The goal is to provide a solar system that will provide 40% to 60% of the annual hot water load. In Southern Ontario, an average person consumes about 50 litres (11 Imp.gallons) of hot water a day. A typical flat plate collector (3 sq. metres) can deliver enough heat to heat 100 litres (22 Imp. gallons) a day. Therefore for an average family of 4, the standard sizing would be two collectors. This is the case providing the collectors face within 30 degrees of Solar-South, at angle equal to the latitude (+/- 15 degrees). Also, the volume of storage must be approximately 50 litres per square meter of collector area. If less storage is used the storage will heat up too quickly and the system will shut off prematurely and/or the storage heat losses will be elevated. If the storage is oversized the water will not get warm enough to displace as much  supplementary heat as it could and savings will not be realized.

If negative performance factors such as collector shading and facing the collectors more than 30 degrees off-solar-South cannot be avoided, it is far smarter to add another collector than to mount them on a rack or structure  at an odd angle to the roof.

Flat plate collectors work as well or better than evacuated tube collectors during the Summer. In the Winter, flat plate collectors will operate as well as evacuated tube collectors on sunny days and significantly better than evacuated tube collectors if it has been snowing. Snow does not melt off of vacuum tube collectors until the air temperature gets above freezing. Flat plate collectors melt off snow even if the air temperature is below freezing - and perform much better than vacuum tube collectors as a result. This is contrary to what retailers of the evacuated tubes claim.

7.) PaybacK/Economics

The payback on a PV hot water systems is better than it is for other types of solar hot water systems. The reasons are: they cost less to install (due to the lower number of labour hours they take to install) and; the zero-maintenance they require after installation. The cost of maintaining solar hot water systems is an industry dark secret. The costs to replace failed pumps, plugged heat exchangers, blown expansion tanks, leaking pipes and decayed glycol (etc) are high and ongoing. There are no such costs with PV water heating systems.

The payback on a residential solar water heating system is extended. The low costs of conventional fuels in Ontario make this inevitable. A very good solar package will deliver up to 10 GigaJoules a year. This currently has a value of about $120 if displacing natural gas. This is equivalent to about $470 worth of displaced electricity(Sept. 2016). There have been a number of subsidies and grant programs available that apply to the purchase of residential solar hot water systems, that used to significantly reduce the costs of an installation. However these subsidies come and go - without them the paybacks would be in excess of 50 years (vs natural gas).

Two components which aren't addressed by paybacks or economics: Green house gas reduction and energy security.