Understanding Solar Hot Water Heating

A well-designed and installed system will meet 50-75% (EECA) of your hot water needs. This can change over different months so this is an average over the whole year. We have customers who claim through out the warmer months that they can be nearly 100% reliant on solar as there water heating source.

As the sun gives out an almost unlimited amount of energy and when the sun shines (cloudless day around midday) there are about 1000 watts of free energy available for every square metre of land. The energy potential is ideal for heating water and to substantially lower your hot water cost.

Solar water heating converts the energy of both the visible and the infrared sun rays into heat whenever there is daylight, although more heat will be delivered on a clear day than when it is overcast. Clear Blue Sky 1000W/sqm, Partially Overcast 600W/sqm, Overcast 300W/sqm, Grey Winter 100W/sqm. As an example there is about 1,412 kilowatts available for Wellington per sqm of collector per year. This means if a collector is 2 sqm then you have 2824 kilowatts of energy production. How much of this is converted into actual hot water storage is dependent on design factors, efficiency of the components, and water usage.

The main component of a solar water heating system is the solar collector. The function of the collector is to capture as much solar energy as possible and transfer it in the form of heat to the fluid in the collector. The heated water or a special fluid is transferred by a pump, or thermosyphon, either directly into the hot water cylinder or into the storage water via a heat exchanger.

Your return on investment is dictated by the overall performance of the system, water usage and available radiation at your site. There are other factors to consider about a well designed thermal system but basically, if you have ample radiation and use it right then your investment will be soon paid for.

Types of Solar Water Heaters

Passive: also known as a thermosyphon systems where probably the first principal discovered for heating water from the sun and collecting it in a storage container. How it works is a solar panel is connected to a storage container which must be higher than the solar panel. As the fluid in the panel is heated from the sun (becomes lighter) it flows naturally upwards into the top of the storage container. This process causes the cooler heavier fluid to sink to the bottom of the solar panel. Over the course of a good day the process will fill the storage container with hot water. As seen in the diagram temperature stratification occurs in the solar panel and in the storage container.

Advantages:

• Simple operation, less components and system cost.

Disadvantages:

• Roof strengthening may be required (can be up to 500kg)
• Maintenance or repairs can be an issue due to access and weather conditions.
• Heat loss from tank is greater due to out door exposure therefore additional electrical energy required.
• Low pitch roof construction will require a heavy duty stand.
• Can discharge hot water on to your roof when over heating occurs during periods of little or no usage.

Active: also known as forced circulation has been around for over 30 years in NZ, and has taken over from thermosyphon as the preferred option. How it works is an electronic controller reads temperature in the top of the solar panel and the bottom of the storage tank. When the sensors reach a pre set temperature difference, a small pump forces the hot fluid in the solar panel down to the storage tank. It continues to circulate until the temperature difference is lowered then stops circulating. The process continues all day like thermosyphon until the tank is full of hot water.

Advantages:

• Can have higher solar yields due to controller intelligence and element management. Some incorporate useful data collection and performance analyses.
• Tank can be located anywhere on your site that is practical for servicing, although efficient water distribution needs to be considered. This is a special advantage if you have internal space restrictions or want to free up that hot water cupboard for more storage.
• Maintenance can be easier.

Disadvantages:

• Can have a higher capital cost.
• Pump has a running cost (although negligible)

Open or Closed Loop Systems

Passive and Active systems can be Open or Closed Loop.

Open Loop Solar Systems circulate your storage water (potable water) directly through the collector and stores it in the tank. This works fine with water that is neutral. Hard or acidic water can corrode copper pipes or cause scale deposition, lowers efficiency and reduces the life span of the system. Open Loop systems are not suitable where frost or freezing temperatures are expected unless some type of freeze protection device is fitted (anti freeze device can be unreliable).

Closed Loop Systems circulate a heat transfer fluid through the collector. This fluid is generally a solution of mono polypropylene glycol (food grade) mixed with water and a metal inhibitor to protect the fluid circuit from freezing and preserve the life of the system. It picks up heat in the collector and transfers it through a heat exchanger into the storage water.

Advantages of a closed loop system are:

• Guaranteed purity of fluid circulating through the system no corrosion.
• Frost protection, glycol is a anti-freeze agent depending on the mix ratio, it will protect your system to very low temperatures.

Fueling Your Home with SUNFUEL

SUNFUEL is the next generation in solar water heating, a complete system designed to operate at peak performance with durable long life components.

Through hard and trying experience over many years Negawatt have chosen the SUNFUEL Solar System because of its performance, design and most of all, extremely high quality. Warranty periods are generally a good indicator of quality that's why SUNFUEL Systems are the longest in the industry.

How It Works

Two temperature sensors one at the top of the collector and the other at the bottom of the tank are measuring the hottest and coldest part of the solar circuit. When a pre set temperature differential is reached, the controller livens the pump to circulate the heated transfer fluid in the panels to the heat exchanger in the bottom of the storage tank.

As hot fluid from the panels is transferred to the storage tank the panels will cool down and become ready to be heated again. This process will last as long as there is enough radiation to heat the panels.

Schematic of how all components are connected

Factors To Consider When Retrofitting Solar

Replacing an existing cylinder with a special heat exchanger tank is the most efficient way of entering a solar system. The solar tank can act as a standard hot water storage tank up until the time that you are ready to install and connect solar panels.

The SUNFUEL solar water heating system can be installed to take any pressure from low to high. In fact if you are happy with your current low-pressure system and need to replace the tank, then the solar tanks can operate at low pressure.

SUNFUEL Components