Solar hot water for the dairy?

February 2012

Comparison of solar hot water services

One of the easiest ways to reduce the amount of energy needed in the dairy is to use the sun to help heat water. The following information is provided to help those thinking about installing a solar system for their dairy.

Solar hot water systems have two main parts: a collector and some form of storage

There are two different technologies for solar hot water collectors; the traditional panel collector style, and the evacuated tube solar collector. From a distance the panels look similar to photovoltaic solar cells, but in reality are actually just glass-covered boxes containing copper tubes, which are exposed to the sun.

Collector Panels

Evacuated Tubes

Evacuated tubes consist of a copper pipe running through the middle of a glass tube which is sealed to create a vacuum. This design prevents heat loss through convection and radiation, so the only heat loss possible is from the tank.1

What does it look like?

FIGURE 1: image of a flat plate collector for a solar water heating system.

Image courtesy of Solarhart

FIGURE 2: an evacuated tube collector for a solar hot water heating system.

Image courtesy of Apricus Australia

FIGURE 3: a diagram of an evacuated tube. Think of a thermos bottle. Collectors are made up of a number of these tubes connected to a manifold where the heat is transferred to the water in the system.

There are many claims about which of the two collectors is the best. It is important to consider a number of things when making a decision about which one to use.

One often mentioned statement is that evacuated tubes are more efficient than flat plate collectors. There are graphs to prove it. However often it is not clear exactly what is being measured or compared. Each of the collector types have an absorbing surface that heats up when exposed to the sun. In a flat plate collector this absorber covers almost the full area of the collector. Therefore the footprint or total area of the flat plate is almost the same as the absorber area. In an evacuated tube collector the absorber is a strip inside each of the tubes. In this format the area of absorber is close to half the area of the total tube system. As a result the total energy collected by a system is also dependant on the absorber area not just the total footprint of the collector. The evacuated tube systems can reach higher temperatures than flat plate systems and are therefore often considered a better option for commercial operations where temperatures higher than home use are required. However it is important to consider how the water heating system deals with times when the temperature gets too high. Some users complain of loss of water in the summer time when the system vents water to control the temperature in the system. Check how the system you are considering controls excessive temperature.

Evacuated tubes will collect more energy in cool and cloudy conditions and this is used as a selling point by some retailers. Remember that in these conditions there is already low amounts of energy available, so will you be gaining a great deal by going for the greater "efficiency" of the evacuated tubes? Put another way – is 75% of not a lot better than 50% of not a lot considering the potential cost to achieve it.

The collector is only one part of the solar water heating system. There must be some form of storage or tank to hold the water being heated. This may be the main tank in the system or it may be a holding or feeder tank that is plumbed in before the main or booster tank.

There are two general types of systems used for storage and to circulate the water being heated. The simplest system is a passive or thermosiphon system. The storage tank is mounted above the collector panel. The tank may be on top of the panel or mounted inside the roof but above the level of the panel. The liquid in the panels circulates into the tank via the thermosiphon effect (as water heats up, it becomes lighter and rises into the tank).

Active or pumped systems have solar collector panels on the roof but the tank is located at ground level (or elsewhere in the building). Hot water is pumped from the panels to the tank. There are more parts to these systems so they are a bit more complex than passive.

FIGURE 4: A flat plate solar water heater that has a tank connected on top of it. This is an example of a passive system with the water storage directly connect to the collector.

FIGURE 5: A diagram of an active system. You can see that it has a pump to move the water around the system from the storage tank to the collector and back again. Some solar water heating system may be described as a split system. This simply means that the storage tank is not directly connected to the collector but located at some distance away from it. It may be an active or passive system so be sure you know what the details are for the systems you are looking at.

Most dairies will need to boost the water temperature achieved by the solar system to reach the temperatures traditionally used in dairy cleaning. This will also impact on the design of the system you will need.

So how do I make the best decision for my business? A good way to compare systems is to compare the cost of the system with the energy output of the system. How many dollars will you spend compared to the output in energy measured in MJ or kWh. Getting that information can be more difficult in Australia than the United States which has a national body2 that rates the systems available there.

Some of the elements that impact on the performance of solar heating systems are the:

• air temperature,
• position of the sun relative to the collector,
• temperature of the water being heated,
• The weather and sunlight,
• amount of water being used and how and when it is used, &
•  the temperature of the water in the storage tank.

Ideally mount the collector facing North. There is a reduction in the total amount of energy that can be captured as the direction moves away from North. Tilt angle should be the latitude of the installation site. The latitude of Melbourne is near enough to 37 degrees. Adding 15 degrees helps in the winter and going 15 degrees lower helps in the summer. Some of the installation manuals suggest that being 10 degrees lower than "optimum" is OK. This makes it possible to mount some systems parallel to the roof. This means that the loss in energy collected is not worth the extra cost of creating a special mounting on the roof rather than just mounting it parallel to the roof.

Consider potential shading effects. About 65 to 70% of solar radiation comes between 9 AM and 3 PM. Trees may not shade the roof area now but will they grow up to cast a shadow later on? What about the location of grain silos or other buildings?

When talking to a provider of a solar system be sure to consider the following issues:

• weight of the units,
• strength of the roof where it will be mounted,
• angle of mounting,
• is north facing possible,
• durability of unit,
• price (including installation), &
• energy output of the unit.

Design and quality of the total system and the installation are also critical to a good system. Make sure your supplier and installer has a track record that you can check out. The best system in the world is no good if not installed and maintained correctly. Always try to install the tank as close as possible to the collectors and as close to the area the water will be used as you can. Make sure the pipes are as insulated well.

Please give some consideration to the items in this document. Do not just think about the amount of energy the salesman tells you his system will save you. There have been many promises that have not been realised relating to solar hot water on farm.

Good luck making good use of the sun that falls on your place.