Solar hot water systems for dairy shed

Flat plate and evacuated tube systems

Capturing heat from solar energy to preheat water for sanitising the milk harvesting plant can save electricity and energy emissions. Two common solar hot water systems are flat plate and evacuated tubes.

The technology

Solar hot water systems pre-heat water using solar energy. The systems have two main parts; the collector and a form of storage. The collectors transfer heat from sunlight to radiant energy to heat the water. The collectors are glass covered boxes containing copper tubes which are exposed to the sun. They can either be a flat panel collector style or an evacuated tube solar collector.

• Flat panel systems can be more sensitive to frost and are generally heavier and larger than evacuated tube systems.
• Evacuated tube systems are generally more expensive than an equivalent flat plate system however the evacuated tube system can heat water to higher temperatures.

The location and climate of the farm will help determine which system is most appropriate.

The storage or tank component of the solar hot water system can be part of the main tank or more commonly an additional holding tank that is plumbed into the system before the main or booster tank. The cycling of water between the collector and storage can be either passive or active.

Evacuated tubes for solar hot water in the dairy (As illustrated in the image above)

1. Cold water inlet

2. Water to evacuated tubes

3. Warm water to solar preheating tank

4. Warm water to existing hot water service

5. Outlet to mains

Photo 1: Flat panel hot water system (image courtesy of Solahart)

Photo 2: Evacuated tube system (image courtesy of Apricus Australia)

Factors to consider

Some of the considerations for optimal performance of solar hot water systems are:

Installation and maintenance – correct installation and mounting is critical to the success of the system. The roof needs to be strong enough to hold the system. The collector should be mounted to face north and alignment away from north reduces the amount of energy that can be captured. The tilt angle should be the latitude of the installation site.

Air temperature, weather and sunlight – the ambient conditions to heat the water will impact the energy savings of the solar hot water system. The higher the water temperature that can be consistently achieved will improve the profitability of the system; keeping in mind the process in place when the temperature is too high in the evacuated tubes system. Frost prone areas are also a consideration for the flat plate system.

Quality of the system – higher quality collectors in either system will produce hotter water.

Retro fit kit – it may be possible to install a collector, pump and controller to an existing storage tank, thus reducing capital costs.

Small Scale Technology Certificates (STC) – ask your supplier if your solar hot water system is eligible for STC.

Victorian Energy Efficiency Target Certificates (VEEC) – if a conventional hot water system is decommissioned you may be eligible for VEEC, thus reducing capital costs.

Before making your final decision, seek advice from reputable suppliers of the technology and speak to other farmers who have installed the technology in their dairy shed.

The energy savings

Table 1: The energy and costs saved in year one with a flat plate or evacuated tube solar hot water system

The energy savings in this analysis were based on the Small-scale Technology Certificates (STC) available on the flat plate and evacuated tube hot water systems. The STC are equal to one megawatt hour of electricity generated or displaced. The number of the STC for the technology will vary according to the location where it is installed within Australia. This hypothetical case study farm is in zone four. The value of the STC is determined by market based pricing, and will reduce the capital outlay of the technology. At the time of writing STC were valued at $27. Without the Small-scale Technology Certificate (STC) rebates the profitability of these systems would change.

Business as usual option

The case study farm heats 700 litres of water for sanitising the milk harvesting plant with an electric hot water service. The hot water system generates 61 kWh of heat per day or 22,265 kWh per year. The cost of heating the water is $3,117 and is the business-as-usual option.

A solar hot water system preheats the 700 litres of water. The flat plate system would save 9,600 kWh or $1,344 in heating costs. The evacuated tube system would save 10,800 kWh, or $1,512 in heating costs in the first year (Table 1).

The capital cost of the flat plate system for the case study farm is $10,550 and with the STC deduction would cost $7,958. The capital cost of the evacuated tube system is $11,038 and with STC deduction would cost $8,123.

Do they pay?

The evacuated tube system was most profitable on the case study farm. This system has a value of $12,053 in year ten, compared to $10,052 for the flat plate system. Both systems would take six years to recoup cash outlay.

The flat plate solar hot water system would be more profitable than the evacuated tube system if the capital cost reduced from $10,550 to $7,981, or $5,389 after the Small-scale Technology Certificates has been deducted.

The reduction in carbon emissions for the case study farm if a flat plate solar hot water system was installed would be 12 t CO₂-eq and 13 t CO₂-eq for the evacuated tube solar hot water system. This represents a saving of 43 percent and 49 percent respectively.