Heating water using solar power is not a new concept. Nearly 2,000 years ago, the Romans built public baths with glass walls that used sunlight to heat space and water. Today, there are multiple ways to employ solar power to heat water. These include solar thermal systems as well as systems that can use solar photovoltaic technology.
Solar thermal systems may not be suitable for every home and family. However, systems compatible with solar PV, such as resistive electric heaters or heat pumps are a great option for guaranteed water heating in most of NZ. Let us take a closer look at both these technologies.
1. Electric Water Heater + Solar PV
A resistive hot water system, AKA an electric water heater, uses a resistive element that converts electric current into heat by resisting the flow of current. It uses materials that are essentially bad conductors of electricity, which heat up when electricity is forced through them. This happens when stubborn electrons in these resistive materials refuse to move, vibrating instead with increasing intensity and becoming a source of heat for the water surrounding them.
Resistive hot water system (E Source)
Electric water heaters are simple systems that typically use grid power to work. Therefore, they can also be powered directly with solar panels. Resistive heaters designed for residential use have power ratings ranging from 1.2 kW to 4.8 kW. For a typical, small to medium family, a resistive heater with a 300-litre tank and 2.4 kW rating is usually sufficient.
Like refrigerators or other devices that regulate the temperature of closed spaces, a resistive heater does not run continuously. Using integrated thermostats, it turns on automatically when the water temperature falls below the specified temperature. In New Zealand’s climate, such a heater may remain operational for 3-4 hours per day to maintain the water temperature. This means it will draw around 8-10 kWh of energy per day.
To power such a device with solar power, based on NZ’s solar potential, you may need around 2-3 kW of solar panels. Now, the cost of a medium-sized domestic resistive heater lies somewhere around $2,000-$3,000. Add to that the cost of a 2-3 kW solar power system, which could be about $6,000-$9,000. This means that a solar-powered resistive hot water system can cost you anywhere between $8,000 to $12,000.
2. Heat Pump + Solar PV
A heat pump is another great option to heat water using solar power. It is slightly more complex than resistive heaters. In thermodynamics, heat pumps are regarded as the opposite of refrigerators. In other words, heat pumps pull thermal energy from one space and use it to heat another, typically smaller space.
Residential heat pumps are designed to transfer heat from the outside to the inside of a house. Like refrigerators or air conditioners, these machines have a compressor and evaporator, and use electricity to operate these components.
Schematic of a heat pump
Heat pumps are popular for being more energy efficient than resistive heaters, partly because ‘moving’ heat is easier than ‘generating’ heat. But the other, more interesting aspect of a heat pump is that it draws less instantaneous power in a short duration. Unlike resistive heaters, heat pumps draw moderate amounts of power for a longer period.
What this means in the context of solar power, is that you can use a smaller system to heat the same amount of water. In fact, even using grid power, heat pumps consume less energy than electric heaters. A well-specified and installed heat-pump water heater would reduce your hot water bill by two-thirds over standard electric water heating.
A decent quality heat pump system may cost you around $5,000. If you are planning to run it with solar, you may need 1-2 kW of solar power, which would cost about $3,000-$6,000. In conclusion, a solar-powered heat pump would lighten your wallet by $8,000-$11,000, which is in the same range as solar-powered electric heaters. Of course, these numbers are indicative, and can change based on numerous factors.
Which One Should You Buy?
Like most other comparisons, there is no clear winner in the fight between resistive water heaters and heat pumps - even when we are comparing them as solar-powered devices. Both technologies have their pros and cons, and a set of your own specific parameters will decide which one suits you perfectly. Here’s a look at some of those parameters:
Cost
As a general rule, electric water heaters are cheaper to purchase than heat pumps. But another exciting general rule says that heat pumps are more efficient, which means they have lower operational costs. A long-term comparison shows that heat pumps save more money in the long run.
But the difference is more pronounced when you bring solar power into the equation. Although the initial cost of electric heaters is lower, they need more solar panels to run. Heat pumps can work with a smaller solar panel system. All in all, the purchase cost of solar panels + electric heater is comparable to that of solar panels + heat pump ($8,000-$12,000). And since heat pumps last longer than electric heaters, they are a clear winner in the cost battle.
Complexity
Heat pumps are undoubtedly complex systems - with one component placed outside the house and others inside. They may also require periodic maintenance, since a refrigerant moves around in tubes at varying pressures.
Electric heaters, on the other hand, are simple devices with working principle similar to your baking oven or clothing iron. Electric heaters are therefore much easier to install and even maintain, if a maintenance issue does come up. Another benefit of resistive heaters is that instead of using a solar battery to store excess solar energy, you can simply use the extra energy to heat more water, turning your electric heater into an energy storage device, albeit one with limited capabilities.
Load Shifting
Like many other modern electronic devices, both electric heaters and heat pumps can be coupled with a timer. In the case of electric heaters, a timer can ensure that the device turns on during hours of maximum sunshine, avoiding or minimizing any grid power consumption. Alternatively, if you have a smaller solar power system, you can also time the electric heater to switch on during off-peak hours so the cost of grid power consumed is minimum.
Heat pumps, on the other hand, consume less instantaneous power and have a more gradual energy consumption throughout the day. For instance, a heat pump may turn on and off fifteen or twenty times in a 24-hour cycle, while an electric heater may only need to turn on 2-4 times.
This means that a timer won’t be of much help with heat pumps, since the device needs frequent switching on and off. A timer can still help maximize solar consumption or minimize grid costs, but it will not be as effective as in the case of resistive heaters.
Therefore, when it comes to load-shifting advantages, electric heaters prove to be much better than heat pumps for use with a timer.
External Conditions
Since heat pumps ‘move’ heat, it is important that the source of heat has enough thermal energy in it (in this case, the surroundings of a house). This is why heat pumps typically don’t work well in cold climates.
Particularly in colder climates, such as that of South Island, a heat pump may not perform to its best potential. It is a good idea to ask the seller if the heat pump works well in, say, a 2-degree-Celcius weather. In conclusion, when it comes to external conditions, an electric heater turns out to be a more reliable device.
Noise
Electric water heaters do not have any moving parts (unless you count dancing atoms inside the coils). Therefore, there is barely any noise when operating a resistive heater. A heat pump, on the other hand, has several moving parts, resulting in a noticeable amount of noise. If you are planning on installing the heater anywhere near, say, your bedroom, an electric heater is a better option.
Climate Impact
Thanks to their higher efficiency and lower energy consumption, it is an almost universal fact that heat pumps are ‘greener’ than electric heaters. However, if you are going to use solar energy to operate an electric heater, the difference is a bit less stark.
That said, heat pumps still remain the cleaner option for two reasons - firstly, electric heaters will need a larger solar PV system, which means more components, which means more mining and manufacturing; secondly, electric heaters draw more instantaneous power, which means it will likely draw some grid power every time it switches on.
Other Options
If, for some reason, both resistive water heaters and heat pumps don’t work for you, here are a few other options you can explore.
1. Instant Electric Heater
Instant heaters are almost the same as regular resistive heaters, except that they have a much smaller or no storage tank. As the name implies, these devices are good for heating water quickly and in small quantities. They are a cheaper option, and often have a better efficiency than their bigger siblings with storage tanks.
However, they might not be suitable for places where power cuts are frequent, or where large amounts of hot water are required. Instant heaters can also be powered with solar power, just as heaters with storage tanks.
2. Gas Heaters
One of the most common types of water heaters, gas heaters have been around for a long time. They have a construction that is similar to electric heaters, except they are heated by combusting natural gas or LPG.
However, gas heaters are expensive in the long term, especially since gas prices keep on climbing, and are often unpredictable.
3. Geothermal Heat Pumps
Also known as ground source heat pumps, geothermal heat pumps have a design that’s nearly the same as regular heat pumps. The main difference is that geothermal heat pumps use thermal energy from below the ground instead of the ambient air. These systems are steadily gaining popularity across the globe, thanks to efficient and clean operation.
However, the construction of geothermal heat pump systems is highly complex, with the need to create trenches for underground heat exchangers. Naturally, the cost of such systems is also significantly higher. Geothermal heat pumps can also be operated using solar panels, and their high efficiency means a smaller solar power system typically suffices.
4. Solar Water Heaters
Wherever the conditions are suitable, it seems prudent to heat water using the heat in sunlight, instead of converting the light into electricity and then into heat.
Solar water heaters use flat plates or evacuated tubes to expose water to sunlight, heating it up for domestic use. Solar water heaters can provide 50-75% of your hot water needs in many parts of New Zealand.
Solar hot water systems were commonly installed 10+ years ago when the Government had a subsidy available for installing them. However, since the subsidy was removed and the cost for solar photovoltaics (PV) has dropped significantly, solar hot water systems have become less common.
Conclusion/Summary
Hot water heating makes up approximately 30% of an average household’s energy bill. Needless to say, using solar power for heating water can slash your energy costs by almost a third while protecting you from rapidly climbing energy prices. Over multiple decades, this amounts to thousands, perhaps tens of thousands of dollars saved.
The main ways to use solar photovoltaics to heat water is to power either a resistive electric heater or a heat pump using solar panels. While heat pumps have a higher purchase cost, they have lower energy consumption, meaning you can cut down on the cost of your solar power system when you use a heat pump.
Of course, electric heaters have their own advantages, such as lower purchase costs, quieter operation, load shifting and simple installation. Ultimately, there is no direct answer as to which technology is better in general. Different families will find different options suitable for their needs.
One thing is clear - powering your water heating system, or for that matter, your entire home using solar power is always a wise idea.
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