Can batteries be installed on solar pumps to increase pumping hours?
In Zambia, there is little to no occurrence of solar pumps with chargeable battery systems. The systems are available from suppliers, but the big contractors that WaterAid Zambia works with do not have any experience installing, operating or maintaining battery solar systems for pumps.
Do you have some examples of such systems used in a rural setting, and any advice on their use?
For me your comments raise additional key questions. Understanding the total yield potential of the boreholes and recommended total pumping time will be critical in the first instance before any technical solution is sought. For me this would indicate whether or not an additional borehole is in fact indicated from this perspective, rather than a pumping technology perspective.
Then assuming the yield capacity is adequate, the next thing to consider is the bigger picture of the long-term sustainability of the overall system. Introducing additional costly components in my view only adds to the potential critical failure items on the system. If we choose to add batteries, have we determined the overall system efficiency and will be end up also adding additional panels in order to store enough energy and/or be able to pump and store at the same time. My fear is that the life-cycle of batteries are significantly shorter than that of the pump and the panels and when they fail they will simply not be replaced as deep-cycle batteries are really expensive.
At the end of the day, taking a wider view and looking at the overall lifecycle costs, the capability of the operators to maintain the systems, etc, I would suggest that investing in additional boreholes or alternative supplies may well be the more prudent decision (I was tempted to add “in the medium term” but in fact I think even in the short term).
I cannot see from the email stream what exactly the details of the project are, but assuming that this is a WaterAid funded project, I would suggest that we undertake a yield assessment, followed by a rough series of scenario costings of the different options (not only capital but a 10 year life cycle costing) which will in my view provide a solid basis from which to choose a route forward.
Just some random thoughts from my side:
a) Ensure that system is used as efficiently as possible: this could be by regulating flow during the day - and thus have some capacity during the evening. Ensure that leakages are minimal
b) Batteries still need charging - this would then be via additional panels? Battery supplied solar systems usually do lengthen the time of the pump: it can run at more stable speed (instead of stop-go-slow-speed-up if the pump is directly dependant on the sun).
c) A temporary solution may be adding pumping capacity using a generator. Though far from ideal, this will allow monitoring: one can check if the boreholes sustain 16hr pumping (and allow 8 for recovery) Run this for at least 3 months during dry period to make reasonable stimate. If they do - then enhanced power will be cheaper than new boreholes.
d) Both generator supply and battery have a distinct risk of robbery.
e) Check if there is anybody already supplying generator power (e.g. for mill). Maybe that person can be hired in (and tariff will need to be adapted accordingly).
f) Whatever the solution is, costs will increase and thus contribution from community as well. Ensure that this is well covered! Maybe 12 lpppd is financially sustainable, but 40 lpppd is too expensive (as battery/generator/new boreholes are needed). That latter point obviously is a tough call.
My impression is that implementers usually avoid use of battery systems when installing solar for rural as they add extra costs and complexity to the system. Another set of things that could go wrong. They usually opt for sufficient storage to ensure there is water available outside pumping times. When we reviewed solar pumped systems in Uganda this was also an issue that came up.
Please see the attached brief from the International Energy Association which references the issue on page 11.
“PVP systems do not require batteries, which are expensive and need a lot of maintenance. In PVP systems, the energy is usually stored in a high‐level storage tank, which feeds the water by gravity to public water taps. The optimal storage capacity is roughly twice the daily water discharge rate.”
Also page 3 of the Practical Action technical brief:
“A typical AC system would also need batteries which require maintenance and add to the cost as the system is less efficient and would need a larger array.”
Thanks Vinny for the good information, and it is good to hear the perspective of these specialists.
Just from a simple perspective, it seems pumping through the night would be critical. In the case of the system in question in Zambia, the costs savings in would be significant. There, supply is an issue as the two boreholes only provide 40% of the demand to the system. Thus, increasing storage will not help unless the pumps can run through the night, and the only viable option I can see is increasing the flow through installing more boreholes – expensive.
I can only think of one situation where this might produce more water than a direct solar pumping system and that is such a bad situation you would want to avoid if possible:
If the sustainable recharge of the borehole is so low that to meet the needs of the community it takes continuous pumping equal to or less than that low recharge rate. In that bad case, adding batteries and additional PV to make up for battery losses could allow for a low volume pump to run 24 hours and produce more water than a pump that flows well above the sustainable recharge of the borehole, but shuts down frequently due to drawdown low water level.
That is, if a borehole recharged at say 1 L/sec and you needed more than 3600 L times the locally available full sun hours (say 4 hours average), or more than ~15,000 L/day then your only choice would be a low volume pump at say 0.75 L/sec pumping 24 hours could give you about 64,000 L/Day. It could work but the efficiency would be so low that the cost of the extra solar panels, controls and batteries (plus maintenance) might equal the cost for another borehole, which would likely be a better solution.
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