Hybrid Solar-Compost Powered Water Heating System (Ethiopia)
As an Eco-Lodge, showers are obviously an important facility which guests have certain expectations about. Our lodging facilities are comprised of local thatched circular huts which are wooden framed with mud rendered walls. We had originally planned to put showers inside the rooms. However, we changed our minds about this due to the potential problems of having a regular water source inside a grass, wood and mud house in our environment, relating to dampness, smells and decay, the latter being greatly accelerated by the presence of termites, where moisture is available in moderate amounts.
Being somewhat budget constrained, we had to develop a design of shower on our site that would be cheap, but also durable on an unstable soil base, easy to clean and maintain and provide adequate privacy. Hence we designed and constructed a simple 3m x 4m shower block with reinforced slab-concrete foundation, tin roof and bamboo screen walls which houses two showers. Three shower blocks were constructed in July-August 2009 and a fourth in December 2010. Prior to this there were only bucket showers improvised using bamboo screen and lining the ground with sand and rocks. This was very unsatisfactory for many guests who demanded an over-head shower, ideally with hot water.
With the new shower blocks installed, most guests were happier about the facilities. However, we still only had cold water. As an off-grid project with only a couple of hundred watts of solar we had to find a suitable way to heat water for our guests. In 2010 we developed our compost heating system which we previously reported on. Very simply, a 200L oil drum is plumbed into the water system on its side and a compost heap built over the barrel.
The good things about this system are:
- You get lots of compost (which we are making anyway – so may as well use the heat).
- the barrel can be at ground level since the head pressure from the main tank pushes water through it (cold goes in at the bottom and hot comes out at the top), so no need have the barrel raised up in the air.
- No need to insulate the barrel – the heap both heats it and keeps it hot.
The system has allowed us to heat water, produce large amounts of compost for our garden and also to establish nuclei of plant growth at points on the site where the showers are located, where it would otherwise be very difficult to establish productive species. All our showers have grey water soakage pits planted with banana, sugar cane, papaya and sweet potato. You can see the heap in the photo above, wrapped in a white tarp next to the shower.
However this system also has several disadvantages:
- The heaps usually remain hot for only five days to a week before they need to be turned or they cool down. This means you have to take the compost off and then put it back on again, i.e. turn it twice, which is a lot of work, or turn it off and make a new one on the barrel, which is also a lot of work and you lose a lot of the potential heat as the first heap heats up again after being turned off the barrel. So the system needs a lot of labour to maintain it, which is fine if you have a lot of labour and need a lot of compost (which we do, in fact) but there are times when we may be busy doing something else and end up with cold showers, which is not fine for our guests.
- The barrels tend to get corroded very fast by the compost and end up leaking quite frequently and it’s amazing that painting with enamel paints etc. only seem to stave this off temporarily.
So we’ve been applying that last little loop at the end of the design process (Monitor, Evaluate, Tweak) to this matter. One of the PC design principles is that important functions should be supported by multiple elements. So we should really have a back-up heat source for our shower water — since it is important for our guests to be able to get a hot shower any time. So I think we’ve conceptualised a good system that can overcome both the problems above without being too technically complex to engineer on site with locally available materials.
The alternative heat source we hit on is the sun, which makes sense in Africa. The advantage of heat from the sun is that there is no labour requirement at all.
- The sun only shines in the day time – so you have to store the heat for night/early morning for as long as 12-14 hours.
- The sun moves about so the heating surface has to be out of the path of shadows or it may be shaded for part of the day.
- In cloudy conditions it may not work at all.
In terms of combining solar heating with compost heating, we have the following points to consider:
- You can’t heat a barrel with sunlight when it’s in the middle of a compost heap.
- Compost heaps are best at ground level.
- A solar catching surface is best at roof level where it will avoid shadows.
- Hot water rises due to lower density than cold water – so a convection loop can be set up which will transfer hot water into an insulated container from a heat source which is lower down than the container itself – hence if the solar heater is at roof height the container will have to be as high or higher.
Hence the system design comes out like this:
Alternatives we considered were to have the container inside the compost heap with the solar heater transferring heat into the container from outside – but this had several flaws:
- The heat of the container may get too hot and cook the compost.
- The heap has to be at ground level so the solar heater would also have to be at ground level making it liable to be shadowed by surrounding vegetation and structures.
Overall this system design seems to combine the best set of positives, though there are some disadvantages:
- The container has to be raised up off the ground and supported at roof level. We can do this on the roof of the shower, but we will have to assess whether this will be structurally feasible or we will have to build a separate stand for the hot water tank (note that the hot water tank must also be lower than the main reservoir or it won’t fill up!).
- The reservoir itself, as well as the up-flow section of the convection loop, must be insulated to stop heat loss between the heat source and the reservoir. Since this system is not likely to produce temperatures which will cause combustion of organic material we can use natural materials for this such as cotton buds (which we have plenty of) though they will have to be water-proofed from the outside so they don’t get wet and go rotten.
The advantages are:
- In the dry season when compost heaps tend to dry out fast, there is plenty of sun, so even if the staff get slack in keeping the heap moist, there will be plenty of hot water from solar.
- In the rainy season the converse will be true.
- The system won’t heat up the compost heap because if the hot tank water gets hotter than the heap, the convection loop will just stop functioning – i.e. the hottest water stays at the highest point.
- Perhaps the best advantage is that the heating element can be plumbed in with a flexible pipe so that it can be moved. This will allow us to turn the heap between two spots and shift the heating element as we turn the heap – hence the heap can be turned two or three times allowing us to get more heat out of a single compost heap without additional labour.
We are planning to implement the system using 1" poly-pipe. The heating elements will comprise 8 x 1" pipe sections which are 1m long. They are joined in a row by 7 x 1" T-joints and 2 x 1" elbow joints as shown below:
The T-joints and elbows are connected by 1" nipples. The element needs to lie at a slant, with the outflow end elevated above the (cold) inflow end. The hot water container will comprise a 100 litre plastic drum – easily available in this area. It will be sealed shut by cementing the lid on with copious PVC adhesive. The fittings for the heating element’s inflow and outflow will be attached by heating them and then melting them into the plastic walls of the barrel. This creates a good seal. Any leaks after this can be sealed with silicone. Separate inflow and outflow fittings will be attached so that the hot water exits from the top of the barrel to the shower and the cold water from the main reservoir enters the barrel from the bottom, without affecting the flow of water through the convection loop circuit – i.e. four fittings will be plumbed into the barrel. Once this is done the 100l barrel will be placed inside a larger 150l barrel and the space between the two filled with cotton buds – which insulate the heat. The up-flow (hot) section of the convection circuit will be insulated by coating in cotton buds wrapped in polythene and sealed with gaffer tape (duct-tape).
So that is the tweaked design for our shower water heating system. We are also planning on developing a new hot water system for our kitchen, which will produce hotter water for cooking with, using the waste heat from our wood-stove. I will tell you about this in a subsequent update.
These are just two of the appropriate technology systems we are developing at our demonstration site and planning to install in April-May during our 4-week internship program. You could be one of the people here to help us and learn-by-doing how to implement systems like this, at our PRI Master Plan demonstration site in Konso, South Ethiopia.
Hope to see you here!