DID YOU KNOW: Solar Water Heaters
08.03.2022
What is a Solar Water Heater (SWH)?
This is a device that heats up water for bathing, washing, cleaning, and other domestic tasks using solar energy. This technology is generally installed on the north-facing roof for southern hemisphere locations to harness more solar energy available to heat water during daytime for storage in an insulated storage tank for use when required.
A Solar Water Heater comprises an array of solar collectors to collect solar energy and an insulated tank to store hot water. During the day, water in solar collectors gets heated and is either pumped or flows automatically on the thermos-siphon principle to the storage tank.
Types of Solar Water Heaters
There are two types of Solar Water Heaters currently available on the market which are:
One is based on flat plate collectors (FPC) and the other is based on evacuated tube collectors (ETC). FPC-based systems are metallic in nature and have a longer life as compared to ETC-based systems made of glass which are fragile in nature.
Both these systems are available with and without heat exchangers. They can also work with and without a pump. Systems without a pump are known as thermos-siphon systems and those with a pump are known as forced circulation systems.
| Evacuated Tube Collector (ETC) | Flate Plate Collector (FPC) |
ETC-based systems are cheaper than FPC-based systems and they perform better in very cold areas. FPC-based systems have an anti-freeze solution and are also good in very cold sub-zero temperatures, although they cost more than the ETC-based systems. However, in other regions, both perform equally well.
Systems working on the thermos-siphon principle are simple and relatively inexpensive. They are suitable for domestic and small institutional applications, provided the water quality is good and it doesn’t have high chlorine contents. Forced circulation systems are generally preferred in industries or large establishments.
Where water is hard and has high chlorine content, an FPC-based system must be used with a heat exchanger as it will avoid scale deposition in copper tubes of solar collectors which can block the flow of water as well reduce its thermal performance. ETC-based systems will not block the flow of water but their performance may deteriorate due to the deposition of salt contents on the inner surface of glass tubes, which could be cleaned easily once in a year or so.
The Flat-Plate panels are however much stronger and more durable than the tubes and a quality flat-plate collector panel will generally enjoy a longer lifespan than an Evacuated Tube collector panel. This explains why FPC geysers cost more than the ETC solar geyser of the same size.
Research findings from ZERA Commissioned SWH Study
In 2018 ZERA commissioned a solar water heater study conducted by the University of Zimbabwe which identified that flat-plate-collector types of solar hot water systems are more cost-effective than their evacuated-tube counterparts when used for solar water heating applications in Zimbabwe. This is because of the higher efficiency of flat plate collectors at the temperature needed for domestic hot water and also because of the generally longer warranty (an indicator of longevity) guaranteed for flat plate collectors.
Typical flat plate collector
Benefits of Solar Water Heater
- A 100 liter per day capacity system suitable for 3-4 people can save up to 1,500kWh annually, depending on hot water used.
- Higher capacity systems will save a higher amount of electricity/fuel while further reducing a remarkable amount of CO2
- The SWH reduces demand on the electricity network and helps reduce load shedding. By using a solar geyser, one can save up to 40% of an average household water heating cost. It is estimated that replacing all the currently installed electric geysers with solar geysers will release approximately 420 MW peak demand from off the grid, thus reducing load shedding as well as electricity imports.
Policies have been put in place to encourage the uptake and use of renewable energy technologies. The National Renewable Energy Policy promotes the deployment of solar water heaters and has put a target of 250,000 solar geysers to be installed by the year 2030. The National Energy Policy of 2012 estimates the number of electric geysers installed in households to range from 250,000 to 300,000.
Solar Geyser performance on cloudy/rainy day
On cloudy/rainy days, you still get warm water as the water gets heated due to diffused radiation available in the atmosphere although it takes longer periods to heat. The system, however, is either connected to an electric geyser in the house or an electrical back-up is provided in the storage tank of the system which is switched on when water is not sufficiently hot. So, you still get hot water all the time even on rainy days.
Standards for Solar Water Heaters
Standards for solar water heaters were published for use in Zimbabwe and are available at Standards Association of Zimbabwe to all stakeholders who need them. The table below shows the list of the solar water heater standards published:
| Standard | Description |
| ZWS ISO 9459:1999 | Solar heating systems–– Outdoor test methods for system performance characterization and yearly performance prediction of solar system |
| ZWS 1017 | Domestic solar water heaters – Mechanical qualification tests |
| ZWS 1016 | Installation, maintenance, repair, and replacement of domestic solar water heating systems |
| ZWS 1025 | Domestic storage solar water heating systems. |
| ZWS 1032 | Domestic solar water heaters –– Thermal performance using an outdoor test method |
| ZWS 1032 | Domestic solar water heaters–– Thermal performance using an indoor method |
| BS EN 12975-1:2006+A1:2010 | Thermal Solar Systems and Components Solar collectors |
| BS EN 12976-1:2021 | Thermal Solar Systems and Components Factory made systems |
Incentives for solar geysers
As a way to promote renewable energy technology, solar geysers are exempted from import duty as provided for through SI 147 of 2010, however, only VAT (14.5%) is payable. This incentive is only applicable to solar geysers without an electrical backup element.
Solar Water Heating Regulations – SI 235 of 2019
The government gazetted the solar water heating regulations SI 235 of 2019 which requires all new buildings to have solar geysers installed or else they will not be connected to the grid. ZETDC is in the process of implementing the regulations and as such, they have amended the connection inspection forms to incorporate the requirements of the regulations.
The regulations allow for exemption of the following premises:
- premises with technical limitations.
- premises supplied with hot water from a cogeneration plant in or proximate to the premises.
- premises utilizing electricity generated from renewable energy and the excess is used to heat water as a dump load; or
- such other premises as the Authority may determine.
Maintenance requirements
Domestic SWH systems do not need significant maintenance requirements. Occasional leakages in the plumbing could be easily repaired by common plumbers. In case the quality of water is hard, scale deposition in the collectors may result over the years. This may require de-scaling with acids. Broken glass may also have to be replaced by the suppliers. If outside exposed surfaces are painted, the paint may have to be redone every 2-3 years to prevent corrosion of the surfaces.
Typical Hot Water requirements
The typical daily hot water requirements are shown in the table below:
| Type of Building Premises | Specific Hot Water Demand in liters per day at 60 0C |
| Domestic residential houses | 30 per person |
| Educational institutions such as colleges and boarding schools | 5 per student |
| Health institutions such as Hospitals, Health Centres, clinics, and similar medical facilities | 50 per bed |
| Hotels, Hostels, Lodges, and similar premises providing boarding services | 40 per bed |
| Restaurants, Cafeterias, and similar eating places | 5 per meal |
| Laundries | 5 per kilo of clothes |
The Solar Thermal Technology Roadmap for Zimbabwe
The Solar Thermal Technology Roadmap for Zimbabwe which is an addendum to the National Renewable Energy Policy targets to reach 0.1 m2 of collector area per capita by 2030 or an equivalent of 2 million m2 of solar collector area by 2030. The projected sector contributions and targets are as shown in the table below:
| Sector | Specific Target | Number of Systems | Estimated solar collector area to satisfy demand | Percentage of Overall Vision |
| Domestic
(Thermo-syphon and pressurized systems) |
Existing electrified urban households (retrofitting) | 300,000 | 600,000 m2 | 30.00 % |
| Existing non-electrified urban households and rural households | 185,000 | 370,000 m2 | 18.50 % | |
| New urban houses | 425, 000 | 850,000 m2 | 42.50 % | |
| Public and Commercial
(pressurized systems) |
Hostels, Hospitals, and Hotels | 4,800 | 145, 000 m2 | 7.25 % |
| Industrial
(process heating and cooling) |
Food & Beverages, Textile, Chemical, Mining, Agriculture, etc. | 350 | 35, 000 m2 | 1.75 % |
| TOTAL | 2,000,000 m2 | 100.00 % | ||
The installations will be largely driven from the domestic systems contributing 91% of the total number of installed systems. Industrial and commercial systems will contribute the remaining 9% of the installations. For household consumption, geysers running on electricity are the most intense consumers of energy, while solar geysers provide the most cost-effective method of providing hot water.
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