Recycling heat from drainwater

We go to a lot of effort and expense to heat the water for our showers and baths, then let all that heat down the drain. What if there was a way to recycle the heat? It turns out there is. All you need is a simple heat exchanger that allows the hot drain water to preheat the cold water that flows into your hot water system or the cold tap of your shower. Unless your drain pipes are accessible, this may work best on new buildings or renovations.

A heat exchanger specifically designed for this purpose called the Power-Pipe is available from RenewABILITY, or in South Africa from Easy Energy.They say it can raise the incoming termperature from 10 to 25 degrees Celsius.

The incoming cold water passes through the copper pipes wrapped around the drain to absorb the waste heat.

Thanks to Treehugger where I picked up the idea (and the image).

These systems are called drain water heat recovery, and you can find more information by searching for that term by clicking this Google link.

Now what I would really like to see is heavy users of hot water like laundries, hair salons, gyms, using heat pump technology to actively pull the heat out of their waste water.

City of Cape Town planning solar hot water finance.

Solar hot water systems give great savings each month, but the upfront cost can be a barrier to folks installing them. Cape Town City is planning to help with a scheme that allows residents to pay off the upfront cost with a small monthly fee added to the municipal rates account.

From the March issue of CityNews:

"The City will make it easier for residents to pay for solar systems. The Eskom subsidy will be used and residents will be offered a fixed repayment plan through their rates account, at a figure equivalent to the present electricity charges for water heating using a normal geyser."

Full marks to Cape Town for this excellent initiative - now please hurry up and get it launched.

If you can't wait, try the Teljoy option covered recently in Resource Mouse.

Santam supports solar hot water

Santam has just announced that it will facilitate replacing conventional hot water cylinders with solar systems in insurance claims. There is more information on the Santam website or by calling a broker. The site does not say how the additional cost (if any) of replacing with solar will be funded.

Review: Save energy on cooking with Wonderbag

I have been testing a new product which reduces the energy required for cooking by insulating a hot pot so it continues cooking when you take it off the stove. The product is Wonderbag by a South African company, Natural Balance. The bag is made with a poly-cotton fabric and stuffed with insulating polystyrene balls. Natural Balance makes the bags in a poverty alleviation project, so there is a second benefit.

I decided to try it with a curry, and I included chicken, vegetables and potato in the mix to test how well the various items were cooked. Being in a bit of a hurry, and keen to really test the system, I reduced the Wonderbag recipe book 20 minute cooking time to around 15 minutes before transferring the pot to the Wonderbag. Six hours later we opened the bag to eat our curry dinner.

The ingredients

Getting started on the stove

In the bag

Opening the pot

Dinner

The result: The curry was delicious, and it definitely took less energy to cook than the normal stove-top-only method. Larger chunks of potato were still a bit firm, while smaller ones were well cooked. I think I did not allow enough time for the big chunks to heat through before transferring to the bag. After 6 hours the food was still hot enough to serve, but any longer and I would recommend some reheating or microwaving.

While the hot pot was nestled in the Wonderbag, I noticed that the sides and top of the bag were not warm to the touch, but the base was slightly. This means that the main source of heat loss was conduction (down), not convection (up).

I liked: The bag accommodated my big pot easily. It definitely saved energy as promised. Preparing the food in advance means that dinner is ready whenever you are, so it is great for guests. It can be a great way to make padkos that will be ready when you arrive at your destination. Many foods, like stews and curries, taste better with time for flavours to permeate. The bag lets you handle multi-dish meals with limited number of hotplates. Another use for the bag is to keep plates warm while the oven is busy.

Limitations: The bag is quite big to store in the kitchen. I am not sure I will always have time/energy to prepare supper before leaving for work in the morning – I can see it working on the weekend though. Cooking probably stops at 2-3 hours as the temperature drops from 100°C when it was being boiled. This limits the total energy savings for a pot of food, and also limits the benefits at altitude.  It would be interesting to experiment with bagging a pressure cooker (15 psi or 120°C), but ours is too big.

The verdict: It has definitely earned a place in our kitchen.

Spreadsheet to calculate payback time for solar hot water

Thanks to http://www.phillipmartin.info for the graphic.

I have put together a spreadsheet that models the following things for a household:

- hot water usage
- electric power consumption for heating this water
- cost of buying this electricity from Eskom by year with projected increases
- savings from using a solar geyser
- cost of financing the solar installation over a period
- a future projection to show break-even date

Here is the Google docs version of the spreadsheet model. Here is the Excel version.

Cells in green are inputs. Most of the defaults are set to "reasonable" values for a South African household. Important inputs to get right are the number of showers and baths run on an average day, the size of the hot water cylinder, and the thermostat setting (usually around 65 °C).

The sample data in the model comes from a colleague. His household uses around 5 showers per day and they have a 250 litre tank. The model tells us they use 395 litres of hot water each day, that this takes 11.8 kWh to heat, and that this costs R214 per month rising to R289 after the June 2010 Eskom increase. Assuming 80% of heating needs are met from solar power, the household could save R171 per month now, R231 after June.

If my colleague fitted a solar panel at an after subsidy cost of R13,000 and borrowed the capital at 10% over 10 years (e.g., he may have taken the money from his home loan), his repayment would be R171 per month over the 10 years, meaning the deal is cash neutral up till June, then he benefits R60 every month until the next increase when it goes up again.

Finally the table at the bottom of the spreadsheet shows that his savings will have completely offset the cost after 41 months, in June 2013.

Note that the Eskom subsidy will be reviewed (reduced) annually so you have a one-year window starting 11 Jan 2010 to benefit from the full rebate.

Eskom increases solar hot water system rebates

Eskom has more than doubled its rebates for solar hot water systems. The change is effective for systems billed after 11 Jan. The new rebates significantly improve the economics of having a system fitted. Eskom have also improved the web page listing the various systems, making it easier to search by province or system attributes.

The cheapest listed system (after installation and rebate) now costs only R2035, although that is a 75 litre low pressure system. There are lots of systems available with an installed cost of less than R10k. You can find links to the information on Eskom's demand side management site.

I have pulled the figures into Excel and plotted total cost against rebate (which is a measure of performance of the system), to highlight the best value systems.

The nine best value systems are highlighted on the graph. Cheaper/smaller systems are lower and left on the graph, bigger/more expensive are higher and right. Details for the nine systems are below. You can look them up on the Eskom website for more info.

You can access the full spreadsheet on Google Docs here.

Come on South Africa, this deal is sweet enough. Fit solar and start doing your bit for the environment and your pocket.

Congratulations to SA Green Blog award winners

Congratulations to Urban Sprout (winner) and all the nine runners up for the 2009 South African Green Blog of the Year awards. You can get all the details here http://www.sablogawards.com/2009/.

The awards for green blogs are:

Best Green Blog

Blogs covering mainly environmental, organic or enviro-friendly related content.

Winner: urbansprout.co.za

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Runners Up:

• mothercityliving.co.za
• carbonsmart.com
• trueblue24.co.za
• tracystokes.co.za
• blog.earth-touch.com
• africancarbontrust.org
• greencars.za.net
• africandiver.com/blog
• thewrendesign.com

Give them a look.

Measuring your energy consumption

I have started writing a blog on sustainability issues, and have written a few posts already, but have only now linked it to my Facebook account. If this is the first time you are seeing it, welcome. Please feel free to post comments.

It is very difficult to manage something you can't measure, so a good first step in controlling your electricity usage is to get a way to measure it. I was lucky enough to be given an OWL energy monitor. (Click the link for local availability). It has two pieces. The first is a sensor (coil) that clips around the live wire bringing power into the house. The second piece is a portable wireless display of your current and cumulative consumption.

The OWL portable display reading kW.

One of the first things we did was to go around the house switching off things until we had a zero reading. Then we went back switching stuff on to see how much power it was using.

The results:

- around 750 watts for the koi pond filter pump

- around 200-300 watts each for the two desktop PC's

- around 100-200 each for the fridge and deep freezers

- occasional very high consumption for kettle, microwave, oven, hot plates, and the washing machine and dishwasher when they are heating.

- thanks to our solar hot water panel, I have never seen the power consumption from the hot water cylinder.

Commercial References

My blog is intended to make it easy to implement sustainability improvements in your own environment. Often this is through products or services that you can buy from suppliers. I don't want this blog to seem like a cheap punt for commercial interests, but I need to give enough information for readers to find the stuff they need.

Also, companies that are supplying good sustainable solutions deserve to be supported.

Solar hot water without the capital outlay

One of the issues many people have with installing solar hot water is the upfront costs. There is a solution at hand, from local rental company, Teljoy's new division Teljoy Solar, who specialize in financing solar hot water solutions as hire purchase.

This is a win/win solution for everyone. Customers get the benefits of solar and can use the monthly savings to pay the rental/installment. Teljoy makes money on the interest rate they charge, and the environment wins as clean solar energy replaces the dirty electricity consumption.

Low flow showerheads

We have just changed the showerheads in both showers to low flow ones by Oxygenics we bought locally from Eternally Solar in Hout Bay. We used the "skin care showerhead with comfort control" in the shower that has a fixed head, and the "body spa skin care" showerhead on the shower with a hose and handheld option.

The fixed head cost R345 and the handheld one was R645. They definitely use less water, and provide a good shower, but tend to be quite noisy if you turn up the pressure. Both are rated at 9 litres/minute.

Reducing the amount of hot water we draw from the cylinder will mean fewer days when the electric element will need to top up the temperature.

The "comfort control" mentioned is a secondary control at the showerhead that allows you to turn the pressure down without losing the temperature balance. For instance if you want to soap yourself or shave, you can turn the pressure down, then turn it back when you are done.

Eskom maintains a listing of low flow showerheads here. The list just provides models and suppliers, no information like flow rates.

Solar water heating

When trying to save power, for most people the quick win is saving on water heating costs. We installed a solar system with two collector panels when the house was built 12 years ago, and they are still working well. For at least half of the year we don't need any extra heating, and in winter we just need to "top-up" the temperature. We use hot water around 6-7 am for showering. The top-up heating is on a timer to come on for an hour in the morning, finishing just before shower time.

The reason the timer switches off the heater before we use water is so that we don't waste power reheating the water when we will not use it for another 12 hours, and there is a good chance the sun will heat it anyway. The rate of heat loss from the cylinder is proportional to its temperature. This way we minimise heat loss. The down side is that water may not be very hot at other times of day.

We also turned the thermostat of our cylinder down to around 45 degrees C, so we don't need to add much cold water to the mix for the shower. Remember, the cooler the water in the cylinder, the less heat loss we will need to replace.

Any other ideas?

Welcome

With fuel and electricity prices rising regularly, and (here in South Africa) the threat of rolling blackouts if we collectively don't save enough power, it is definitely time to face the facts:

- most of us use far too much of earth's resources
- current usage is not sustainable - future generations will inherit a badly degraded planet

None of this is news to any of you, so why the blog? The blog is a place to explore ways to reduce resource consumption. Over time I hope it becomes a catalogue of resource or energy and environment saving ideas and discussion of the practical application. I am looking to showcase examples of real solutions ordinary people can do now.

Why the title? Well a resource mouse is one who uses only a tiny (mouse size) share of resources. So the blog looks for ways we can each reduce our resource footprint.

Please feel free to post constructive suggestions or questions.

Ro Richardson