Please read this first...

If you want to know what I'm on about in the shortest time then please read the introductory first post and my current action plan. Comments are very welcome. And if you like this blog, please tell a friend. Thanks!

Saturday, 29 December 2007

Pipes and Pumps, part 2

The good news is that in terms of reducing our use of town water, this project is definitely meeting its objectives! We've done piles of washing and started flushing the loo again with wild abandon. I even hosed off the concrete along the back of the house after doing some top-dressing of the lawn. With the weather we've been having lately (lots of showers) we've been unable to get the water level more than about 1200L below capacity, and as of this morning the tanks were full again. Makes me wish there was some way to use that water for bathing as well.

However... all this water abundance has come at the cost of increased electrical consumption. Of course I knew that some electricity would be necessary to power the pump, but I've discovered two ways in which our setup is far from optimal.

First, the pressure switch. That blue gizmo on top of the pump which monitors the pressure and decides whether the pump needs to be running or not. Courtesy of my power meter I have discovered that the cost of monitoring the pressure is apparently about 15W continuous power consumption, or the equivalent of one rather bright CF bulb shining 24x7. In most homes that would be considered negligible, but in my home that's slightly more than a 5% increase in our average daily energy use.

The obvious solution to problems of that sort is simply to switch off the device when it's not needed. And sure, the pump needs an appropriate switched outlet to plug it into. But that would only solve part of the problem.

Three out of the four taps fed by the pump are used in such a way that it makes sense to switch on the pump before use and switch it off again afterwards - doing a load of washing, filling the tub or a bucket, using the hose and so on. The toilet, though, is a different matter.

With a toilet you need the water to flow into the cistern after your, um, "activity" is completed, and then only for a relatively short time. Can you imagine being required to flush, wash hands, walk to the laundry, flick the switch on, wait 30 seconds then flick the switch off again? Can you imagine teaching young children to do that? No, a simple switch is not going to suffice here.

The next most obvious suggestion is to install a second switch in the toilet, wired up in a 2-way configuration like many light fixtures are - such that flicking either switch up or down will change the on/off status of the light, or pump in this case. It would alleviate the walking part of the problem, but you'd still be left with the waiting.

What I'm hoping to do is a variation on that idea using a logical "OR" operation and a time delay switch. This switch is normally in the "off" state. When you push it in, power is allowed to flow but only for a minute or so until the spring-loaded mechanism returns to its initial position. So you push one button on the cistern to flush the toilet and then another button on the wall to refill the cistern, then you walk away (to wash your hands, of course). Either the laundry switch OR the toilet time delay switch can be independently activated to provide power to the pump as required.

Actually both of those switches will feed into the exterior switched socket that the pump is plugged into, so the correct logical expression is: (laundry OR toilet switch turned on) AND (external switch turned on) -> (a powered pump).

I foolishly bought all three switches today without first asking about the prices. Total cost $106. Could have saved twenty bucks if I hadn't asked for the laundry switch to have one of those little red lights in it to show when it's turned on. But anyway...

At current prices of 21.455 cents per kWh (15.455c regulated tariff 11 plus 6c GreenSwitch renewable energy premium) I'm going to have to have the pump switched OFF for about 33 thousand hours - nearly four years! - to recover the cost of the switches alone. But then I'm still predicting that energy costs will rise rapidly over time, and hopefully these switches will be in use for a long time after they've paid back their cost.

The moral of the story is to carefully check the electrical requirements of the equipment you're installing in a bid to save water. Thus endeth part 2. Next time I'll look at the other, far more important aspect of the energy efficiency equation when it comes to pumping water around: the pipes.

(Got any ideas about how to design a really efficient tank/pump system for home? Please share in the comments.)


Ian said...

The power vs water trade-off is currently getting a bit of attention on the ABC: When saving water costs energy.

TB said...

Wow, you've got a great memory, mate!

Thanks for the link. I'm not surprised at the results, and I'm glad somebody's been formally researching it.

At our place I think we've got a fairly optimal setup: the pump is only connected to the washing machine and toilet, which require reasonably large amounts of water at full speed fairly infrequently. We only provide power to the pump (and its pressure-demand sensor that sucks 15W continuously) when it's needed.