P.L. Gas and Solar Ltd have been active in the solar industry in Rotorua and the Central North Island for the past five years. In this time we have supplied and installed many domestic and commercial systems, so we feel we have a good understanding of what works. In Rotorua and around the Central North Island, the best systems have the best frost protection and overheating management as well as the greatest performance. We have a wide range of packaged systems that cover most domestic applications. To view these systems click here.

We realise that understanding the different systems and their appropriate applications can be confusing. As a starting point, let's define the main components of a solar water heating system:

Storage

This is essentially your hot water cylinder or HWC. This consists of an insulated tank that the water heated by your solar collectors is stored in. Storage HWC's come in many variety's and are combinations of the following: Mains pressure, medium pressure low pressure horizontal,vertical tanks with heat exchange coils or tanks with no coils. Mains pressure tanks are made of either stainless steel or mild steel with a vitreous enamel coating. The stainless steel tanks need to be made of duplex grade stainless steel and are much better at resisting corrosion and high temperatures than enamel steel tanks. Medium pressure and low pressure HWC's are made from copper. These copper tanks often have a very long service life however the amount of water delivered to the taps may be less than some peoples expectations, also some modern tapware does not work well with less than mains pressure hot water.

Storage HWC's can either be horizontal or vertical depending on the type of installation. Horizontal tanks have greater heat losses due to the large surface area that faces upward. However they make up for some of these losses as they are usually installed with systems that do not require a pump and solar panels that are very close to the storage cylinder which reduces pipe losses. Vertical tanks suffer less from heat loss and have better stratification properties (less water mixing when the tank is in use). Solar systems using vertical tanks generally require a pump and may have greater pipe losses pumping water to and from the collectors if the distance between them becomes to great (less than 7 m is ideal). Some storage HWC's have heating coils fitted to them to allow them to be used for either indirect solar systems, were the water in the solar collectors differs from the water that comes out the taps. Wet-backs or central heating boilers also need heat exchange coils fitted in the HWC to use heated water running through the coils to heat the surrounding body of water.

The best choice of system depends greatly on your situation and the room available to best fit a Storage HWC. An in-home consultation prior to receiving a quotation will help the solar installer advise you as to which system will best suit your needs.

Collectors

Most collectors fall into the category of either flat plate or evacuated tube collectors.

Premium flat plate collectors use low iron prismatic glass as the outer glassing a copper absorber plates with a tinox absorber coating for collecting the sun's energy. Panels with these specifications keep their performance in lower sunlight conditions compared to lower cost standard efficiency flat panels.

Premium evacuated tube collectors use either heat pipes or u pipes within the tubes and heat exchange fins to transfer the heated glass surface to the water to be heated. One key advantage of evacuated tubes is their insulation which provides frost protection.

Types of Frost Protection are as follows:

Frost Protection

Drain Back

This is the best overall way of achieving failsafe frost protection and also the best form of overheating management. The main concept behind drain-back systems is the fact that there is no water in the solar panels unless there is heat to be recovered from the panels and returned to the storage hot water cylinder. These systems require a heat exchange coil either in the main hot water cylinder or in the drain-down tank. The drain-down tank is a small auxiliary tank used in a drain-down systems that is used as a reservoir to store the volume of water that gets pumped up to the solar panels when required and serves to set the resting water level of the system. When the solar pump is not running there is no water in the panels, the solar pump will not run unless the panels are hotter than the hot water cylinder. This means that in practice there will never be water in the panels unless the panel reach a minimum of at least 15 degrees Celsius. If there is a frost outside the pump will never switch on and the water in the system will remain safe inside the insulated drain-down cylinder. On days of clear sunshine when your hot water cylinder reaches its set temperature the solar pump simply turns off at a set maximum temperature and the water drains out of the panels back to the drain-down cylinder effectively turning the system off. Drain-down systems are by far our favoured solar solution and they can be used on both new and existing hot water cylinders with or without wet-backs. They have lower maintenance than systems with Glycol (anti freeze) in them and have less over heating issues than tube systems and they fail safe as opposed to active frost protection systems.

Glycol (closed loop)

Glycol is an antifreeze used in solar systems that is non toxic. Glycol systems require a heat exchange coil fitted into the storage hot water cylinder. P.L Gas and Solar use Glycol systems when we cannot achieve the constant gradient that the drain down pipes require to operate correctly. Glycol systems work effectively and their only real down side is that after repeated heating cycles the Glycol becomes corrosive and requires replacement every 1 to 5 years depending on the manufactures recommendations to prevent damage to the system. This is an added cost that can be avoided if a drain-down system can be used.

Evacuated Tube

Evacuated tubes are inherently frost tolerant as the water in the system is protected by the insulation of the manifold at the top of the evacuated tube collector. In situations where we need frost protection and the system is a thermosyphone system: ie uses natural convection created by hot water rising and the HWC is above the solar collector rather than a pumped system, evacuated tubes are the perfect choice. Over heating is a problem for evacuated tubes however so we need to be careful how we size the systems. Evacuated tube systems are also expensive relative to their performance and we can install a Drain-down with high efficiency flat plate collectors that has better performance and is cheaper than any evacuated tube system that we currently have access to.

Active (pumped) Frost Protection

Active frost protection relies on the solar controller to turn on the solar pump to circulate water through the solar collector when it is below a set temperature. 4 degrees is a common set point. This prevents the water freezing in the panels. This system is cheap but does not fail safe. In the event of a power cut and frost combined frost damage to the system is likely to occur. Frost valves are often used to overcome this problem but these are un reliable. Back up power supplies can also be used however we believe by the time to take steps to cover come these systems short falls you may as well have just installed a fail safe system in the first instance. P.L. Gas and Solar Ltd will only sell these systems in coastal areas that only experience very mild frosts.

Government Funding

Visit the EECA website for the latest government funding options.