sol hot water installed on tile roof

A detailed comparison between solar thermal and solar PV.

An Apricus ETC30 collector has 2kW peak output for 4.4m² collector, operating at 60% – 70% efficiency at domestic hot water temperatures.  This compares to 2kWp photovoltaic (PV) solar system covering 13m² of roof and operating at up to 20% efficiency.

The very high energy output to area of solar thermal system is important as we try to maximise renewable energy generation for buildings from their limited roof area.  This applies to all homes and is most significant in buildings with a smaller footprint, such as town houses, apartments and hotels.

In relation to the solar heating discussion, my opinion is that it is a better solution than trying to get the solar PV to handle the hot water demand. I also trust Marcus from Apricus to design a fit for purpose system rather than an oversized one that will end up having surplus energy and hot water. 

In the load analysis report, I prepared, we considered a daily hot water demand load of 2.8kWh using a 250L heat pump. Annually, hot water demand using a heat pump is around 9% of the living house loads but with solar heating, you will be able to further reduce it to approx. 7% of the house demand – this will progress you toward the +5% energy target for LBC.

It is also worth mentioning that solar power and solar heating peak at the same time of the day – when the house electricity demand is fairly low. Hence, it makes sense to me to store energy both in the battery system and in the hot water cylinder to be used during peak demand hours (morning and night).

You are likely to have a battery system that is capable of delivering 2.5kW peak power, and considering electric hot water cylinder elements are typically rated 3kW, the battery will not be able to deliver the power even if it has 5kWh stored in it. Using the battery to support a smaller element say a 2kW, means that you will rapidly discharge the battery for hot water duty rather than using it for peak demand times.

Is it a good choice of technology?

A 300L hot water cylinder requires 15.7kWh of energy to heat from 15’C (Auckland average water temperature) to 60’C.  A 300L cylinder is the absolute minimum size if we are to store enough solar energy during the daytime for the hot water use of ultimate building occupancy of 4-10 people.

A solar PV system will deliver a daily average of 3.6 – 3.8 of it’s peak capacity, i.e. 1kW peak = 3.6 – 3.8kWh average daily generation.

Therefore to deliver 15.7kWh to heat water to 60’C = 15.7 / 3.8 = 4.13kW peak PV array size.  This is array is just to heat water.

A lot of the time this system will be oversized because it will generate more than 3.8 times peak.  But on other days (winter & other cloudy conditions) it will not heat the water to the required temperature because 3.8 is an average figure.  In winter and during cloudy weather this will be nearer 1 or 2.  So the array needs to sized somewhere near average for the year.

In addition, a hot water cylinder generally has a 3kW element in the tank.  Therefore, a PV system must deliver at least 3kW at any one time to ensure that the balance is not made up by the mains.  So a 3kW PV array dedicated to water heating is the minimum size.

An alternative element size is 2kW.  If this is used to reduce the PV array size then time multiplied by energy to give temperature becomes an issue, i.e 15.7kWh (to heat cylinder) divided by 2kWp = 7.85 hours of heating at 2kW peak output required.  This is feasible during summer but unlikely at other times of year.

Is it cheaper to use a solar PV system?

Solar PV panels deliver varying amperage depending on solar insolation levels.  A 3kW element draws 13 amps.  If 13 amps is not delivered the balance will be made up from the mains.  A smart cylinder controller can be used to remedy this situation and “chop” the current, matching supply to demand.  Like a thermal MPPT (Maximum Power Point Tracking).

So a 3kW PV system costs would cost at least $7,500 assuming some efficiencies with existing panels.

A smart cylinder controller costs around $1,000 installed.

So heating water using solar PV will cost around $8,500, being conservative and accepting the fundamental energy dynamics above.

As a comparison, an Apricus 30 tube solar hot water system will cost $6,200 installed, all things being equal (standard installation, etc)

How reliable is solar thermal for heating hot water?

Not all solar thermal systems have been created equal.  Therefore quality and long term support are important.

Apricus have been designing, manufacturing and supplying solar thermal systems in NZ and Australia for 15 years.  We feel that we’ve had plenty of opportunities to iron out any issues.  All the systems come with a 10 year warranty on everything on the roof, plus 15 years on frames and manifolds.  We expect a 20 year life span from the evacuated tubes and heat pipes.  At this point they will need to be replaced with a new set, with another 10 year warranty and an extended life of an additional 15-20 years.  At the end of this we expect the whole collector will need to be replaced.

The balance of the system (pump, controller, insulation, valves) are from high-quality suppliers with standard warranties.  We expect and have experienced 10 year minimum life span from these.  This compares with the life span of the other PV system components such as inverters which will last around 50% of panel life span.

Will the system over heat?

Apricus systems have been designed to cope with over temperature situations. 

The Apricus controller will turn off the pump when the cylinder has reached a maximum set temperature, isolating the collector.  The collector will continue heat and any high temperature water is isolated in collector and will not return to the cylinder.

They don’t vent or release any water.  They don’t release or vent steam.  They don’t dump hot water.

The flip side to solar thermal systems being able to generate higher temperatures than solar PV systems and electric element is that the reserve of solar heated water is greater during periods of high solar energy.

An Apricus solar hot water system will regularly heat a stainless steel hot water cylinder to 80’C.  As opposed to 60’C set point for electric thermostats.

This increases the amount of solar energy stored as hot water.  This in turn reduces the amount of electrical energy needed for boosting as the hotter water last longer when tempered down to 55’C for consumption.    

This provides an additional 150L of water (338L at 60’C vs. 488L at 80’C, when delivered at 55’C)