Solar Hot Water - Is It Worthwhile?

Posted on May 14, 2017
Tags: building, green, tomar

For a long time, I thought that using solar water heating systems heating water for baths, showers etc was an obvious thing to do, even in the UK. Surely, it should make a better case for itself in Central Portugal?

However - I'm not so sure.

Let's consider this study: domestic hot water consuption

The mean household consumption has been found to be 122 litres/day, with a 95% confidence interval of +-18 litres/day.

It has revealed that the only one of these factors influencing consumption is the number of occupants. The mean energy content has been found to be 16.8 +- 2.2 MJ/day. Energy content of water delivered has been subjected to the same statistical analysis as the flow data, and has also been found to depend only on number of occupants.

analysis of the average temperature rise of water as it passes through the heating system (derived from the initial cold feed temperature and the hot water delivery temperature) shows a value of 36.7C, significantly lower value than the 50C currently assumed in BREDEM. On this sample of dwellings BREDEM would over-predict energy consumption by approximately 35%. The current temperature difference is based on an assumed cold water inlet at 10C and hot water delivery at 60C. The discrepancy with the measured result is due partly to hot water temperatures lower than assumed, and partly to cold feed temperatures higher than assumed.

Consider also this (from 2013): At homewith water

Each person uses about 142 litres of water each day. The average household uses 349 litres of water each day Hot water use contributes ?228 to the average annual combined energy bill and emits 875kg of CO2 per household per year.

Water temperatures Temperature (C) Temperature Rise (C)
Mains 13.4 -
Shower 41.0 27.6
Basin hot 55.0 41.6
Kitchen sink hot 55.0 41.6
Bath 44.0 30.6

The specific heat capacity of water is (kWh / L / K) 0.00116

I find these figures slightly surprising - I would have thought that my own use is more than 1/3 hot. (I am comparing average household hot use, with average household use). But I work in an office 5 days a week - and the cooking and cleaning and toilet there does not contribute to my view of what I use at home.

On this basis, the hot water use per person per day is about 40-50l.

The usage pattern for Portugal would presumably be similar - except that the mains supply temperature should be higher - average annual ground surface would appear to be ablut 15C. I'm surprised that it is so high in the UK - it may indicate that a lot of use relating to hoses for garden watering and car washing are concentrated in the summer.

I suggest that the temperature rises for Portugal should be lower - I thonk 5C is probably justifiable, but I will play safe with about half that. Most of the hot water demand has shifted to showers.

The corollary is that:

So the number of kWHr per year that we need is:

Annually, that's 588kWHr - let's call it 600kWHr per person.

If we use the energy calculator here lusosol then the cost of direct electric heating is estimated at €0.125 per kWHr - so the total per person is only €75 per year.

A more conservative view is here enerpor which has electricity at €0.17 per kWHr. They might both be right - there are tarrifs that favour off-peak use and a thermal store can mitigate, albeit at some cost itself.

If there is just two people most of the time - the spend is only €150 per year on heating water on the low estimation and €205 on the higher. I will use the latter, higher figure.

That makes it remarkably difficult to justify buying a more efficient heating system, unless the system is also handling space heating and can provide water heating as a side effect.

A generous view suggests that we can provide three quarters of the hot water needs through a solar hot water system - saving just over ?155 per year, compared to a direct electric heating system using a resistive element (assumed built into the cylinder, or provided as an electric combi-boiler in place of a cylinder). We still need a backup heating source.

If there is a pellet boiler, log burning boiler, or heat pump in place then the savings will be a fraction - less than €60 per year.

To put things in context - the savings of €155 is comparable to the cost of having a boiler serviced annually. If there is no main boiler for space heating (for example in a passivhaus situation) then direct electrical heating is easily the cheapest solution, because the capital cost is so low for an immersion heater. In the casde where there is partial occupancy, the case against high capital expenditure on installing a device that needs servicing is even stronger.

Let us suppose that we require a payback in 15 years, and that to make the calculation easier we will match the cost of funding capital expenditure to the expected rate of inflation on fuel costs, and disregard them.

That means that:

If this is a new installation, then these budgets reflect the extra we could spend over a basic facility. If it is an enhancement installation then they represent the replacement or enhancement of the existing system and it may be harder to justify integrated systems that include a water cylinder if one exists already.

We can immediately see that if there is a low cost heat source (where it is a given, for space heating needs), then there is no feasibility for solar hot water to be worth bothering with. It cannot be done with professionally installed commercial equipment for €960 - this is effectively the cost of a basic thermosyphon system, without delivery or installation. Building and installing DIY is also tricky in that budget unless you have a really low opportunity cost.

In the case where there is no other heat source, it is worthwhile, based on an integrated system like this one: ferco drainback Even then the case is somewhat dependent on the costs of installation.

If we consider a holiday home, then we have only partial occupancy - so the total available savings is reduced based on occupancy fraction. Often the occupancy may be when the solar fraction is 100%, but the total cost of water heating is still low in absolute terms compared to the cost of installation. We may be forced to use solar thermal by regulations - in such case it makes sense to buy the cheapest compliant solution and use direct electrical heating as a supplement.

For the current house we have an oil boiler - which is not low cost to run - but the occupancy fraction is low. We also have a hot water storage tank that is currently unused since we use the combi boiler.

In principle it should be possible to utilise it with a blend valve so the boiler is not used if the tank contains hot water, using something like this:

It seems a lot of money and effort.

Perhaps worse, there is a possibility that even with the combi diverter (and Caleffi make an alternative) the possibility of Legionella remains. See this interesting article from founder of Viridian Solar.

That would suggest that a pasteurisation system should be arranged using the existing boiler coil or the immersion heater. Losing the use of the boiler coil means that loading the solar energy becomes harder - we'd need a SolarSyphon or similar.

Conceivably the issue is less pressing in Portugal - especially if the collector is oversized and uses the evacuated tubes. That would clearly push towards a drain-back design since we have deliberately over-sized the collector. Even with stratifying technology like the SolarSyphon or the Copper Industries' HotHead there is no guarantee that water is not stored at a dangerous temperature before it is drawn off.

Perhaps the answer is to use a thermal store with a heat exchanger and never store hot water. Or - just say no to solar thermal, and spend the money on a bigger PV installation. A good flat panel collector with 2m^2 area is 95% efficient and costs about €700: the same money will buy about 600W of raw PV capacity, which is more flexible.

Indeed, the cost of all the components to 'solar enable' the existing water tank exceeds the cost of the 500W autoconsumo kit here which could indeed contribute to running the immersion heater in the tank - and other electrical devices around the home like the fridge and freezer.