Reducing Cost of Water Heating

Archie

Our home hot water tank holds 30 gallons. We used 2612 units to keep this heated all year. At current prices of 29.37c per unit the annual cost is $767.14 or approx 7.2 units per day at cost of $2.10.

At present we cannot use our own generated power to do this heating, our daily surplus gets exported for which we are paid 7c per unit in summer 10c in winter.

How much hot water do we actually use? Probably average 2 showers per day, two sinks of dishes, one bucket in the laundry. Plus numerous short hand washings and plate rinsings.

So, are there cost-effective ways to reduce the amount of energy needed to achieve this heating and/or to divert our solar surplus to this task?

iSmart

Hi,

I suggest looking at the iSmart Hot Water Controller. Its a hot water controller that can be retrofitted to your existing hot water cylinder. Its Kiwi designed and manufactured.

HOW DOES IT WORK?

•You can programme what times you use the hot water most

•Calculates if there is enough hot water and only heats up when required

•Easy to use LCD display touch

•7 Days worth of data

•BOOST MODE - when extra guests turn up or you want to top up your hot water

•HOLIDAY MODE – your hot water can be safely turned off while you are away

•LEGIONELLA PROTECTION - boosts automatically once a week to kill off any bacteria

I installed this in my own home about 4 months ago. on average per month we were using 632kW

My last bill came in only yesterday actually and our kWHr usage was 437kW

We have installed this now in approximately 255 homes across NZ and have are finalists in two categories for the Sustainable Business Network Awards 2015. Business Innovation and EECA Energy Management

Archie

iSmart thankyou for your suggestion.

You have made savings equivalent of our total usage per month! Well done.

Does timing of the heating really make a lot of difference? I would have thought that heating a tank from cold would take more power than just keeping a warm tank topped up. Perhaps I am wrong, but how could I ascertain this?

Kex

Why can't you use your excess generated PV power?

I have a system (DIY) which is based on the EmonPI + PV Router from the OpenEnergyMonitor project.

Basically I have a little Arduino sitting next to my distribution board which has current/voltage sensors for reading the amount of energy I am importing/exporting.

This little device can then monitor the system and when I am exporting energy, i.e. PV generation is greater than household usage, a signal is sent to my HWC to turn on.

The clever bit is that the Arduino monitors the energy flow in real time and is able to cycle the HWC very rapidly (a couple of times per second). By doing this the system can gain a state of equilibrium so that only excess power is diverted to the HWC. This means if someone turns on the kettle and all of a sudden my household usage spikes, the HWC is immediately disabled until there is an excess again.

There is potential wear on the HWC element by switching it this rapidly but it is a simple resistive element so this should be fairly robust.

I have been running this system for about a year now and it works like a dream. I have some temperature probes setup in my HWC so that I can disable the PV diverter once it hits 65 degrees, and then I start exporting and collecting the credits.

It is not a black box system, and I wouldn't expect most to be keen to try something like this, but it shows what can be done.

I wish I had some more resistive dump loads I could divert excess PV to, since the buy back is so low these days. And in summer the HWC is usually heated up by midday so I have a load of excess PV I could be using elsewhere...

Archie

Kex this sounds clever. Did you install this system yourself? Have you calculated how much you save in relation to the installation cost?

Archie

Kex In answer to your question, our HW has always been on a separate, controlled, tariff. I now see no advantage to this, because not only is the tariff the same as our low user rate, but it runs only on imported power.

However, I don't want to make changes that cost more than I would save.

Right now I am experimenting with the completely low tech solution of turning it off the cylinder completely and requiring everyone to use the second shower. I will not be popular, because they have to go outside.

Also, to my frustration, because we have two power phases this shower draws imported power too, but the tank is only 25litres so I expect it should cost a lot less to heat.

iSmart

Archie

In regards to your query 'heating a tank from cold would take more power than just keeping a warm tank topped up'.

An important part of this management is stratification control; Within the same hot water cylinder it is possible to have both hot and cooler water.

The hot water will sit on top of the cooler water without it mixing. Cold water enters into the bottom of the cylinder and rises as a layer while hot water
is drawn off the top. Traditional style heating of the water will decrease and eventually destroy this stratification, unless you have a Smart Controller which will manage this heating to your advantage.

The Smart Controller knows where this stratification line is (the technical term is a thermocline), and to what degree the different water volumes might need heating to be useful for you. Only when absolutely necessary will it heat, and not before and even then use just enough energy to ensure you never run of hot water. So no waste of energy usuage.

This way a lot less heat (and money) is lost through the cylinder walls compared to keeping fully heated water in the entire cylinder. This can be quite substantial as testimonials from our customers confirm.

There are other methods of using stratification control with an iSmart Hot Water Controller for even greater savings if you are using, or intend to use, special energy sources such as a solar pv or solar thermal.

The idea is you only heat the hot water when you need it. With the iSmart Controller set your Profile Settings for the time when you want hot water available.

My power bill before the iSmart Controller
June to July - 667kW/month
July to Aug - 580kW/month
Aug to Sept - 321kW/month ( installed iSmart)
Sept to Oct - 325kW/month
Oct to Nov - 437kW/month

Here is a video link about the iSmart Hot Water Controller.

https://www.youtube.com/watch?v=sUVMglsy8J0

www.ismartcontroller.co.nz

I've been in the home energy industry for over 20 years and my business partners have been in the hot water management industry for about 25 years. So together for the last two years we have been developing this technology, to help reduce the biggest hungry power device in peoples households.

Kex

Archie I did install the system myself, but only after consulting an electrician friend of mine. It uses a chunky Solid State Relay with a big heat sink for switching the HWC.

Haven't really worked out how much it is saving me - hard to do as I also had solar hot water evacuated tubes installed when I built the house, so on a sunny day I am getting both the tubes and the HWC element heating up the water - doesn't take long to heat my 350l cylinder to 65 degrees!

Check out the www.openenergymonitor.org site/shop. They have a few different products, all open source and reasonably priced. Very good gear for measuring power usage as well as graphing etc.

Archie

Kex That seems like a large tank, so it's great how quickly you can heat it. How many in your household? Do they use hot water all day long? I do. But I probably don't NEED to.

Kex

Archie Yep got a large tank since we went with the solar evacuated tubes. There are only two of us currently, with a third on the way. Probably average 3 showers a day, cooking and cleaning etc.

Archie

Kex Hey, exciting that you have new person on the way! With a growing family, a large tank is a good idea. Ours is shrinking. When I had a household of five, we sometimes ran out of hot water in winter due to the ripple control kicking in. But now we are shrinking, down to three at the moment and our small tank is enough.

Hopefully the even smaller tank will keep up, and I am considering a water urn or something for the kitchen, plugged in to the same phase as our PV panels, which were installed 3 years ago.

skamp

Archie Look up the ImmerSun from Enasolar.

iSmart

Only works if you have Solar PV. The iSmart works with or without...

skamp

iSmart @Archie has solar PV

iSmart

skamp Yes there are a lot of people who don't have Solar PV

skamp

Yes, and you Senztek/iSmart are experts in water heating control.

The need to divert excess solar PV seems to be the orginal post's query.

The ImmerSun discovers whether any electricity is exported to the grid, and diverts this to the hot water cylinder. it does this by altering continuously the draw by the cylinder element to match the excess PV, something that the iSmart unit cannot do - let me know if that is incorrect?

iSmart

skamp

The iSmart is not a diverter. It does not impedence match the load to surplus power.

However all present diverters do not measure the cylinder temperatures and do not know if you have too much or too little hot water.

To get around this you can either manually boost if you run out of hot water or use the onboard timer to heat for a pre-determined period of time.

This can lead to the user running out of hot water or boosting too much and wasting energy

As an alternative we propose that since one needs to heat water anyway, then heat at the most likely time of the day for solar gain and then have the iSmart avoid heating outside those times as much as possible while still maintaining enough hot water.

Also we cannot see how any diverters presently on the market comply with NZ law for Legionella protection

Another option is an iSmart controller and a diverter together on a dual element cylinder that solves ALL the issues.

Kex

The Immersun is very similar to my DIY setup from what I understand. Although it might use a different technique to vary the HWC load?

skamp

Kex ImmerSun uses pulse width modulation (PWM) - does your unit do that? Or is it a voltage chopper or something else?

(I am not a supplier of the ImmerSun)

Kex

skamp It just has a SSR which is switched very quickly (faster than 1Hz) to try and equalise the import/export from the PV.

skamp

Kex Is there a smart meter where you are?

Kex

skamp Yes - in ChCh - with a smart meter with import/export meters and day/night registers.