How to Improve the Performance and Life Time of Solar Water Heating Systems
So what are the most common failures of the majority of solar water heating systems?
1. Water quality as can be hard, soft or acidic
2. The wrong type of hot water tank (vented or unvented)
3. Imported components not suited to your countries needs
4. Solar collectors imported that do not come with a EU certification
5. The wrong plumbing pipe and fittings can cause corrosion
6. Hot water and energy waste due to overheating and no pipe insulation
7. The final design and positioning of the collector and tanks
Where to start
• Always start with researching the countries water quality
Water quality affects the life of the system, the need to service system regularly due to build up of lime scale if hard water is supplied. Water supplied through reservoirs is best. If the country uses desalinated water, this is acidic and can eat away copper very fast.
• Learn all about the types of hot water tanks manufactured or imported
The type of water tank selected, if not from the collector manufacture and a split system depends on whether the system is vented or unvented and water quality of the country. The 2nd issue is the material used in the production of the water tank, the inlet outlet diameters and what type of heat exchanger coils are inside the tank.
For all in one Thermo Siphon vented/unvented systems, mainly manufactured in China and Europe (Greece, Cyprus, Israel, Spain and Portugal) with a choice of evacuated tubes fill with water or all glass heat pipe tubes or flat plate collectors, will all need to be installed with thermostatic mixer valves, to avoid scalding. These types of systems should never be installed in countries with water shortages due to water waste, waiting for the hot water to arrive where needed inside the building.
An unvented pressurised hot water tank should never be allowed to heat up hotter than 65c (A UK safety standard for all manufactures)
The problem with a unvented hot water tank heated by solar, is you cannot stop the sun hating the water except if it rains. Many water tanks exported from China, I have one safety valve (TPR) which opens at 90c to release the pressure in the tank, with up to 20 litres of boiling water being dispersed some where
An open vented hot water tank, with a feed and expansion cold water vessel can be heated safely to 90c
• Educate yourself of the differences between flat plates, water filled tubes and heat pipe collectors from local manufacture or imported.
The decision of which type of collector to recommend for a split system to marry up with the selected water tank, depends again on the all round year weather conditions in your country and what the potable water is to be used for and how often during the day.
If you are in a hard water area where there is lime scale, the use of flat plate collectors needs to be always installed as a split system.
Flat plate collectors are bulky and heavy to carry, if to be placed on to a roof. The majority of flat plate collectors come with soft copper pipe absorbers, enclosed in an insulated box with a single sheet of toughened glass.
Flat plate collectors are less flexible in sizing to hot water tank volumes, as only supplied in 1-2 sizes (2m2/2.4m2), while tube collectors can be sized to exactly the matching size of the hot water tank (1 tube per 10 litres)
The two most common choices of tube collectors is the water filled glass tubes and heat pipe tube collectors.
With the water filled type,(always supplied with a vented stainless steel cylinder with holes for the tubes to slide into) if one of the tubes explodes or is broken, all the water in the tank flows out and continued to flow out, unless the cold mains water to the feeder tank is shut down.
The heat pipe tube is supplied as a separate collector for split and thermo siphon installations and also supplied with a unvented pressure cylinder together (no need for a pump) A heat pipe condenser can give you first degree burns if open to sunlight inside a tube for more than 2 minutes.
For countries with months and months of hot weather and blue sky days, I do not ever recommend this type of closed pressure system, due to my experience in Spain and Cyprus.
Heat pipe collectors are 30% more efficient that flat plate and unlike flat plate, provide a higher temperature during cold cloudy days.
• Understand the difference between closed loop and open loop systems
A closed loop system is when there is a solar coil heat exchanger in the hot water tank located at the bottom of the tank. This type of system was first developed in Europe and the USA 50 + years ago, to prevent water in flat plate collectors freezing.
Oil was used before the introduction of Glycol (antifreeze) mixed with water.
As with all closed loop systems, you need an expansion valve, air release valve, filling loop, circulating pump and a controller, all adding to a higher cost and replacement over a period of time. Where there is no chance of freezing, glycol is not needed, as long as the plumbing system is all copper and brass or stainless steel coupled to a copper or SS tank.
An open loop system, means the water heated by the solar collector is heating the water in the hot water tank direct
This is mainly applied to thermo siphon all in one system with open glass tubes filled with water, or heat pipe tube collectors coupled to a open vented cylinder, where potable water is supplied from.
Water passing through copper pipe or stainless steel is very safe, as long as there are no galvanised or iron pipe fittings used in any part of the system.
Solar Speedflex Co Ltd is just one of a handful of established manufactures in the world who has proven over the past 25 yrs, that you do not need to use a closed loop system design, as long as the hot water tank is specified as a Multi Fuel Thermal Store (Designed and developed in the UK since 1991)
A full detailed explanation will follow on about Thermal Stores, as the ONLY real future for service free, unlimited hot water at mains pressure through a open vented water tank.
Heating water DIRECT from a solar collector is 30% more efficient and 50% less cost than a closed loop system with an internal coil located at the base of a cylinder.
• And lastly and the most important, look at all the types of plumbing pipe, fittings and insulation tube that is being sold to installers
I have kept the plumbing together of the tank and collector to the end and the delivery of cold water from the utility and out of the tank to where it’s required at a safe temperature
It does not matter, how fantastic the tank looks or the efficiency of the collector, as the majority of failed systems within 6 months from the installation is related to the low quality plumbing pipe system.
One of the reasons I began to invest in developing a quality solar hose plumbing system, dedicated to the solar water heating industry since 2007, was because of what I had seen at the Haining SWH test centre in China
The reason why the majority of SWH systems fail, outside the tank and collector is related to the wrong type of pipe, to many pipe fittings, leaky fittings, a mix of different pipe materials and a system not designed to maximise hot water transfer with minimum losses.
I am always amazed at the lack of insulation used or insulation tube like NBR, which will break down due to the UV from the sun even in the UK.
If the reason for leaving the pipe connected between the tank and collector without insulation is to do with cost (which the customer is paying for anyway) perhaps somebody should calculate the heat loss between the collector and tank and from the tank to where the hot water is needed.
The introduction for the first time in Africa of our Speedflex Solar Hose brand, with leak free brass fittings, will revolutionise the way all solar installations are installed across Africa.
Article written by
Eric Hawkins (Renewable Energy/Outsourcing Solutions Consultant)
Henbest Close, Wimborne,
Dorset, BH21 2EX. UK