Photovoltaic (PV) power and solar hot water are two types of solar systems for the dairy. PV power uses panels to generate electricity which can be used, stored in batteries, or sold to the electricity retailer. Solar hot water transfers heat from the sun to water passing through a collector.
PV power:
PV panels generate Direct Current (DC) energy from solar radiation. While they generate more electricity in summer, they still generate power in winter because it’s light, rather than heat which is important.
An inverter, or inverters, converts that DC power to 230 volt AC, and this is fed into the dairy switchboard. A control system can be integrated into the inverter so the farmer can monitor performance from a computer. If batteries are included in the system, the inverter must be battery-compatible.
A single “string” inverter can be mounted on the dairy wall, connected in series to all the panels. DC current generated can be up to 1000V, so special switch gear and conduit to minimise the fire risk is needed. If one panel is shaded by trees or clouds, the system’s output will be compromised.
Alternatively, a micro-inverter is fitted to each panel, so DC to AC conversion occurs at the panel, and 230v AC current flows from the inverter to the switch board. Micro-inverters produce slightly more power and the system isn’t affected by shading. They last longer so have better warranties, and in many cases cost no more than a string inverter.
An import-export meter is installed by the local electricity retailer, so surplus power can be sold back to them. Batteries can also be installed to store surplus power, minimising export and allowing the farmer to use more of the power generated each day.
Off-the-grid systems, where the dairy is not connected to the local supply, will usually only be feasible where the cost of getting local power to the dairy is prohibitive. An expensive bank of batteries is needed to store power, and a generator back-up is needed.
On-the-grid systems effectively use the electricity network as a giant battery. When inadequate power is being generated from the solar panels, power is drawn from the grid. If excess power is generated, the excess is exported to the grid. Some retailers won’t buy back power, but there are retailers in all areas that will. They pay less than the wholesale price for power, and this varies between supply retailers. The system should be designed so that a minimal amount of power is exported and the farm consumes almost all that it generates. The economics of a PV system is affected by the buy-back price only if a bigger system than needed is installed. It’s better to undersize the system so power exported is minimal.
Solar panels are mounted on the dairy roof, or on a frame on the ground.
A building consent won’t be needed for a PV installation unless it affects the roof structure, but installation must be done by a registered electrician and inspected by a registered electrical inspector before commissioning.
PV systems are generally maintenance-free. Panels are cleaned by rain, and the lack of moving parts means little wear and tear. Panels have a 25-year warranty on their power output. After 25 years their power output might have reduced by about 20%. String inverters have a life of about 10-15 years depending on quality, while micro-inverters should last 25 years before they need replacing.
Costs are specific to the dairy’s electrical consumption and the system installed. The more reputable installers will analyse the dairy’s power requirements over at least 12 months, taking into account daily power use patterns.
They design and quote on the most effective system, predicting annual savings, payback time, and return on investment before the farmer commits.
While power saving is the main focus, there can be some revenue for power exported to the grid and for some there will be the feel-good factor as the dairy reduces its carbon footprint and does a little bit towards reducing climate change.
“Solar PV installations typically have a simple cash payback period of around 10 years, with a system life of 25 to 40 years”, Martin Diprose of McNae Electrical Solutions in Palmerston North says.
“But because farmers can claim depreciation at a rate of 16% pa on the farm solar installation, the investment can be recouped in 5-7 years. And the return on investment is in the order of 8-12%, after tax.
Solar water heating:
Heat from the sun is transferred to water either through a flat-panel or evacuated glass tube collector.
Flat-panel collectors consist of pressed stainless steel sheets through which the water flows. Solar heat is conducted into the panels to heat the water. Flat panels have been around longer and are more efficient in very hot climates, like Australia.
Evacuated glass tube collectors consist of glass tubes. Each tube has a smaller one inside it with a vacuum between them, like a thermos. The inner tube has a black outer surface to maximise heat absorption, and a copper inner surface to help trap the heat. Within the inner tube is a copper heat pipe extending up to a manifold which contains the water to be heated. The heat acting on the heat pipe boils the fluid in it. The vapour rises to heat the water in the manifold, condenses, and sinks again down the tube to be reheated, so repeating the cycle. Evacuated glass tubes are more efficient in lower temperatures and cloudy conditions. They are light and easily installed.
Systems more suited to a house have a water cylinder mounted above the panel or tubes. A thermosyphon occurs, with the hot water rising from the panel or tubes to the cylinder, to be replaced by cooler water. These systems have no pump or control system, and are generally too small for farm dairies, but might be suitable for the smoko room and shower.
Installations more suitable for farm dairies are connected to the hot water cylinder or cylinders, and have a controller and circulating water pump. The controller monitors and displays the water temperature at the top and bottom of the cylinder and at the tubes. It controls the element in the cylinder. It controls the pump, turning it on if freezing is possible, and off if the water is getting too hot. Water temperature in the collector can reach 80C, with the collector temperature reaching more than 100C. It also prevents cavitation in the pump by monitoring the pumping time. Standard dairy hot water cylinders can be modified for this type of solar water heating.
The solar panels or collector can be mounted on the dairy roof or on a frame on the ground. The controller and pump are mounted near the hot water cylinder, with a non-return valve downstream from the pump. An adjustable flow-meter is fitted to control flow at about 2 litres/ minute.
Solar water heating installations must meet NZ Building Code standards and a building consent is needed.
The government website energywise. govt.nz lists solar water heaters which meet their Energy Star criteria for efficiency, warranty, and conformation to NZ design and construction standards.
Evacuated glass tube collectors withstand hail, but breakages are possible. The glass tubes can be easily replaced if broken and are quite cheap. There is minimal maintenance associated with running a pump and electrical componentry. Warranties may be specific to the components of the system, with some collectors having a ten-year warranty and the other components like controllers and pumps having their manufacturers’ warranties.
Like PV systems, a solar water system should be quoted not only on cost, but also on savings, payback time and return on investment. This will depend partly on hot water requirements at the dairy, including volumes, temperatures, and times. Once-a-day milking and once-a-day hot washing are more conducive with solar water systems.
Other applications:
Solar-powered electric fence energisers have been readily accepted. Cullimore Engineering of Ashburton make a solar-powered rectangular yard backing gate. Three solar panels charge batteries that power the 24v DC backing gate motor. It is radio-controlled from the dairy, and the low voltage and lack of cables extending down the yard eliminate one source of stray voltage.
The current dairy climate has put the lid on capital spending. But even in the heady days of $8/kg, solar installations were not common. This was possibly because farmers were more production-focused rather than cost-focused. Capital was invested in run-offs and feed to increase production rather than cost-saving devices. With a drop in payout, costs become more important. Investing in cost-saving technology can help when times get tough.
Componentry costs have come down, installation costs remain similar, and technology has advanced. For most solar systems, this has meant a better system for the same capital investment. For example 260 or 270W panels may be installed instead of 240W units.
Batteries are expensive but prices are coming down. They might become more feasible so farmers can store their excess power, rather than selling it at a relatively low price.
“Around the world, the exponential growth in PV uptake is fuelled by steadily falling module prices,” Dr Justin Hodgkiss, a senior lecturer in chemistry at Victoria University with a special interest in solar technology, says.
“Uptake is naturally led by sunnier places and spreads to less-sunny places as prices continue to fall. The vast majority of today’s PV installations are made from silicon technology that was established over 20 years ago. Today, printable PV materials are being developed in research laboratories, offering a route to significantly lower manufacturing costs for future PV modules.”
