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:)

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 thermal 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 1800's. It is now very common in other parts of the world. Japan, Israel, Greece, Australia, Germany 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. Up until recently the cost of conventional fuels has made using solar unattractive financially. We do have a climate that's tougher on solar systems than most places (wind, 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 20 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! Soon you will be able to buy CSA certified solar collectors and 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. 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 about two hundred 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.

 

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 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 90% 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 thee 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.

To summarize:

Flat Plate collectors (FPC)

vs

Evacuated Tube collectors (ETC)

AVAILABILITY Many Canadian & US manufacturers of flat plate collectors. many more distributors. No North American manufacturers of ETC tubes. All are made in Europe, Australia, China and elsewhere.
DURABILITY FPC use tempered glass - much tougher than ETC annealed glass tubes. Tempered glass will withstand hail & vandalism much better than annealed glass. Vacuum in ETC is difficult to maintain -especially in some designs (with metal to glass seals). Site assembly of ETC makes them vulnerable to assembly quality issues.
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 to ETC. ETC are superior for high temperature applications (70+ Celsius, 160+ Fahrenheit) such as space heating using baseboard radiators and industrial process hot water or steam.
COST FPC are usually significantly less expensive per square metre than ETC. FPC maintenance is simpler and less expensive.
WEIGHT For a given collector area FPC are usually much lighter.
ADAPTABILITY North American FPC are built so they can be drained completely. Most 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.

 

3.) Seasonal vs. Year-Round

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 late Spring (Approx. May 24) to early Fall (Approx Sept. 30). What we consider to be a seasonal design, 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 hot 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 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!

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

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 is currently being used by an off-shore manufacturer of an evacuated tube solar system. They use heat-trace cable on the piping and have an electric element in the water storage tank - which is also outside in the freezing weather. Aside from the significant electrical cost to prevent the piping and tank from freezing during the Winter, this design runs the significant risk of being totally destroyed (with possible collateral damage to the building or house it's mounted on) if there is a power-outage during a severe Winter storm. This approach makes sense when used in much milder climates where freezing is seldom an issue. It does not make sense in Ontario - these systems will fail -badly.

Another 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 used) 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 can 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, 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 isn't 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, it 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 4 to 6 years.

Some antifreeze solutions are toxic - mainly because of the corrosion inhibitors added to them to lengthen their service life. For this reason it is required by code in many locations, that they only be used with special heat exchangers - typically double-walled units that can't allow the antifreeze to leak through a crack into the water being heated. This requirement means that either more expensive or less efficient heat exchangers must be used.

5.) Electric vs. PV powered solar DHW pumps

Information coming soon.