Direct PV solar hot water buyers guide

Solar panels on a house roof
You don’t need a lot of roof space for a Direct PV hot water system, although you will need more than for a solar thermal system. Image: iStock/Douglas Cliff

Solar hot water can dramatically reduce your water heating costs, but what if your house isn’t suitable for a conventional solar water heater? There is an alternative that can be used almost anywhere you have solar access, as Lance Turner explains.

Regular solar water heating uses the sun on collector panels or tubes to heat water. In very simple terms, the water to be heated or a water-based heat transfer liquid flows through the panels or tubes and back to the tank, usually through a heat exchanger, transferring heat from the panels into the water in the tank.

Solar water heaters come in two common types—close coupled and remote coupled. A few decades ago, the close coupled system was the most popular. These have the collector panel and tank mounted on the roof, with the tank above the panels. The heat transfers from the collector to the tank using a form of natural convection called thermosyphoning – basically, warm water is less dense than cool water, so rises up through the panels on its own. For evacuated tube type collectors, the transfer process is a little different, but the basic format is the same—the tank is mounted above the collector, both of which are on the roof.

Obviously, this requires a strong roof structure as a filled close-coupled system can weight close to half a tonne. As a result, the remote-coupled system became popular, with the tank on the ground and just the much lighter collector on the roof, with water circulated between the two using a small pump which is switched on and off by a pump controller.

Conventional solar water heaters have some basic disadvantages. Close-coupled systems are simple and reliable, but need a strong roof. Remote-coupled systems are prone to pump/pump controller/temperature sensor failures, which only become apparent when you receive your next energy bill and realise that your hot water has been coming from the backup element for several months!

Another issue with both types is the need to be able to locate them close to the main points of hot water use in order to reduce heat losses through pipe runs. Of course, this applies to all storage-type water heaters, but for solar water heaters it can become more critical if the water heater and/or collector location is unable to be located close to those wet areas. For close-coupled systems it can mean considerable pipe runs, especially in multi-storey buildings, and for remote-coupled systems the pipe runs between tank and collectors can become excessive and require careful insulation to reduce heat losses—remember, the pipe run is actually twice the distance between the tank and collector—from tank to collector, and then back again.

Aside from those issues, solar water heaters can be very effective, especially in higher insolation areas. In warmer climates it is possible to gain 90% or more of your total hot water needs from a well sited and installed solar water heater.

But, what about those situations where pipe runs are excessive, or the roof simply isn’t strong enough for a tank on the roof—often a problem with modern “matchstick” houses? Sure, the roof structure can be reinforced, but that adds to installation costs.

There are a couple of alternative systems that can be considered. The first is a heat pump water heater. A good one can reduce water heating costs by 75% or more compared to a resistive element water heater, but they are expensive, and in today’s closely packed urban housing blocks, finding a location that is both close to the wet areas but far enough away from the neighbours so that you don’t annoy them can be difficult in some situations. Also, heat pumps, being complex mechanical systems, do tend to have shorted lifespans than simpler systems—I have personally heard from many dissatisfied heat pump owners, and reviews of many heat pumps seem to show a high level of unreliability and short lifespan on some makes and models.

So, if your motto is “simpler is better”, and you already have a large solar PV system, you might consider a solar diversion system. These simply monitor the solar generation and electricity consumption in the home and when the former exceeds the latter, the excess is sent to a resistive water heater. The main downside with diversion systems is that they don’t work in a blackout—they rely on the PV system’s grid connect inverter, which will turn off when the grid goes down, unless you have a backup-enabled energy storage system, of course.

But, there is an even simpler system that has been gaining ground in the solar hot water market in recent years, and that’s direct PV systems.

What is direct PV solar hot water?

Put simply, a direct PV system uses a dedicated solar array that is used to power the element in a water tank directly. The system is independent from the mains grid and is not grid interactive, and so can be used on any home with good solar access, including off-grid homes.

Because water heaters are generally designed for AC power and solar panels produce DC power, direct PV systems have an interface between the two in the form of a dedicated inverter or controller. These can be purpose-designed inverters that provide AC to the tank (particularly for systems designed to be retrofitted to existing tanks), or they can be DC power controllers that send DC to either the existing tank elements or a retrofitted element of specific specifications.

Some systems come as a separate controller, while others come as a complete system of controller and tank.

Either way, the controller simply takes the DC from the panels and converts it to AC or chopped DC and feeds it to the storage tank’s heating element(s) just as if it was mains power. The only difference is that the available power varies with solar insolation—but resistive elements care not one iota about that, they simply convert all power fed to them into heat.

Of course, as you might expect, a direct PV controller also operates as a maximum power point tracking inverter, just like a regular grid-connect inverter, to extract the most available power from the solar array.

For times of low insolation, most direct PV controllers also have a mains power input for boosting, just like a regular solar water heater, and this can be implemented in various ways. However, the solar input and the mains input are generally kept separate by the controller, so there is no grid interactivity happening. This is why such systems often need twin tank elements—one for the PV power, one for the mains backup.

This is an excerpt from the Direct PV buyers guide. Read the full guide in Renew 163.

Author
Lance Turner
Lance is Renew’s Technical Editor. He lives in Tasmania with his partner Mia, two dogs, four ducks, four sheep, a cat and many chickens.

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