Sustainable Households, Systems Insights, Real Data

Rainwater Harvesting is often the 'heart' of a sustainable house system

Rainwater Harvesting is often the ‘heart’ of a sustainable house system

By Professor PJ Coombes and Michael Smit

When most people talk about sustainability they talk about and analyse separate elements, a rainwater tank, solar panels, recycling, orientation, cross ventilation. How much does that element cost, what will that element produce, how much will that element save?

Considering elements in isolation is very attractive, it reduces complexity and ‘noisy’ data. Engineers do it, economists do it, that IS how most of us do it. But it is not real. We don’t experience a house as a row of appliances, or a city as a list of buildings, or a forest as lots of individual trees. We experience these as complex places. What we experience is the net output of the whole system. And that is what we should be measuring.

We were having a bit of a grizzle recently, trying to work out when opinions became more important than facts in this country, and of all things we started to talk about water pumps for rainwater tanks supplying sustainable households. These household often include water efficient appliances, rainwater harvesting, kitchen gardens and urban trees.
One quite obvious point that most people grasp is that when you use a pump for rainwater the energy use in the house increases. Obviously if you did not have a pump, and then you get one, and you run the pump, you will use more energy right? This logic is completely sound until you measure it. We measured energy consumed at one of our houses with a rainwater pump and found a 12% decrease in energy consumption from 15 kWh/day to 13 kWh/day. Then we fixed leaking appliances and energy use reduced to 10 kWh/day. An overview of the sustainable house is provided at the Performance of a sustainable house over a decade

A whole lot of people may be feeling cranky at this point, what smart arse nonsense have they come up with now? Maybe that is why people resist data – it is upsetting and inconvenient when the numbers do not support your understanding or the story you are trying to tell.

Firstly let us be clear about what we are measuring. Are we measuring the energy the pump is using? Well we could, but it is not very useful is it? When we pay our electricity bill we do not only pay for the pump. We pay one bill for all of our electricity use for the whole house and that is the amount that is important to us. That is what we measured and across the whole house there was a greater than 12% decrease in energy consumption. Importantly, the house, people and surrounding environment are not a collection of separate components and behaviors – it is a linked system.

But this result does not seem to sense? How could using the pump save energy in other parts of the house? When you measure the whole house, which is a system, you are measuring all the interactions within it. Change the performance of one component of a system and the state of the whole system changes in ways that can be hard to predict. One difference between a pump moving rainwater and mains water from the water grid is the pressure and therefore the flow rates of water in the house. A rainwater pump moves water at about 20-30 litres/minute and mains water at about 40-80 litres/minute. So water is flowing more rapidly out of the tap when using mains water. The flow provided by a rainwater pump is slower. Now imagine that you are heating that water with an electric water heater. You will only need to use a little less hot water to make a considerable energy saving on the water heating and the rainwater pump might be pushing through 10% less hot water than the mains water. That factor alone might account for the measured difference in water use.

There are a further set of possible technical problems with methods for measuring energy intensity of rainwater pumps. Measurement of electrical resistance at the pump may be affected by electrical currents across the whole house and provide misleading answers. Secondly measuring an instantaneous rate of energy use is misleading. Regular measurements of both water and energy are required to create a curve which can be integrated to derive the actual energy vs water use of the pump. Not doing this will overestimate energy use by a factor of 5-10. Remember we are reporting energy use for a volume of water that is used in long term planning. Finally you need to take water balances and energy use through the meter (not a metered appliance) over a long period of time to understand the actual energy balance. For example some reports suggest that rainwater pumps operate for 24 hours a day which is clearly not correct.

Our use of water in houses is at quite low flowrates, around 4-8 litres/minute, especially if we are using water efficient appliances as shown in Figure 1 from our monitoring of sustainable houses throughout South East Queensland.

Figure 1: Flowrates for water use events in a sustainable house

However mains water is being delivered at 40-80 litres/minute and many of our appliances may be operating inefficiently and using more energy as a result. Other than reductions in hot water we were not entirely sure what actually makes up the 12% decrease in energy use. We think the reductions in the household energy balance is created because flowrates provided by the rainwater pump is more closely matched with how we actually use water in and around the house and that leads to more efficient outcomes. It may even prevent or reduce the incidence of breakdowns.
However, we recently upgraded the rainwater harvesting system in one of our houses and restarted a detailed monitoring study. We stumbled on some other issues that impact on the water and energy efficiency of houses with rainwater harvesting. The mix of rainwater and mains water use on each day is shown in Figure 2.

Figure 2: Daily mains and rainwater use from the current monitoring programme

Firstly we were concerned about that our new rainwater switching device was sometimes selecting mains water for household use when rainwater was available in the tank. And we observed that our average daily energy use had increased to 15.5 kWh/day during the period to January 2015 – patterns of our use of other appliances during that period may also influence this result (such as air cooling). The efficiency of the rainwater harvesting system had reduced and the overall energy use increased. We then discovered that the plumber had almost closed the tap leading to our toilet cistern (this is common practice for new houses) – this was making the pump work harder including constantly restarting to supply the toilet water use.

We also found slow leaks in an outdoor tap and our toilet cistern. Fixing the leaking tap and fully opening the tap leading to the toilet cistern in December 2015 provided dramatic improvement in the efficiently of the rainwater harvesting system. Finally replacing the leaking toilet cistern with a 5 Star fixture in February 2016 further improved the efficiency of the rainwater harvesting system. The rainwater pump was no longer working hard and restarting – our average daily energy use reduced to 10 kWh which is a 40% reduction in energy use – and our use of appliances may be a little different – we are now analysing the details . Leaking appliances and a closed tap increased the energy use of the rainwater pump. So the energy and water efficiency of the system we call a house are linked.

So, rather than grizzle, we wanted to provide some useful, effective advice that comes from real data and understanding our houses as systems, not looking at appliances in isolation. This data is based on comprehensive analysis of a large number of houses over 15 years and, not to put too fine a point on it, not funded by water utilities trying to sell water.
1. Use Rainwater harvesting efficiently with mains water. ‘Efficiently’ means using rainwater for toilets, washing machines and outdoors and it means always using rainwater first and only using mains water when the rainwater runs out. 1,000 and 2,000 litre tanks will deliver significant savings but 5,000 litres seems more optimal. Using rainwater for hot water will be more efficient again.
2. Using water efficient appliances: the three most important ones are the shower, the toilet (dual flush) and the washing machine (front loader). The variable factor is the garden which is a whole topic in itself. Other small but effective changes are flow restrictors on taps and a device to divert water to your water tank while you wait for the water to get hot. These two factors together will deliver savings of about 90 kL or nearly half the average household potable water use.
3. Fix leaks. This is a massive problem. About 2% of houses have a leak that is nearly half their normal annual water use. Back to our friend the rainwater pump though, a leak will cause the pump to cycle, meaning that it is operating outside its energy efficient range and depending on your configuration the house will use much more mains water than it should. Once leaks are fixed and taps are opened, switching devices operate at 80-90% efficiency and halve energy use.
4. For our more technical ATA readers, if you are on rainwater, fully open the tap to the toilet cistern and adjust the pressure settings of the pump to household flow rates.

Imagine that your whole house is spinning, like a giant toy, the faster it spins the more water and energy you use. Inside your house all of your appliances are also spinning, some fast and some slow. When one gets faster, like turning on a fan or watering the garden with large shady trees, another one might stop, as you switch off the air conditioner. When you change one component of a system, the state of the whole system changes in ways that can be hard to predict, spinning faster or slower as a whole. This is how our houses actually work, but it is a little more complicated. Our appliances can operate efficiently or inefficiently. An air conditioner, or a pump, will very efficient when it operates in its normal operating range, but if you switch it on and off often or make it work at very high speeds or very low speeds, it will use a lot more energy for the same result. So imagine some of the spinning devices are spinning blurs, because they are being asked to do things they are not designed to do, and this uses more energy and makes the house spin faster. Now imagine that your house is spinning ALL THE TIME, 24 hours a day and 365 days a year for more than 50 years. A tiny increase or decrease in one appliance spinning is, over time, going to add up to a lot. Small changes, like collecting the rainwater from a small rain shower, a couple of hours sunshine on a solar cell, shower water running into a garden bed, using a little less water every time you turn on a tap, flush the toilet, turn on the shower, these small changes go on and on and completely change the behaviour of the whole house. Unfortunately it also works the other way, a leaking tap, an old fridge, the air conditioner eventually add up to big increases in energy and water.