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Richard Blanchard gravatar image

The assumption is the medical centre is either not electrified or may be connected to the electrical grid, but has intermittent connectivity. The most overriding requirement is ensuring the electrical loads can be met by the generation of the microgrid and storage. Furthermore, the generation should be optimised for meeting the worst-case situation.
Electrical loads can be calculated from the power rating of the device multiplied by how long the device is required to be used. Power (P) is measured by the volts (V) times the current in Amps (A) required P=VxA The unit is in Watts or W. An old 60W incandescent light bulb uses electricity at 240V and a current of 0.25A so 240x0.25 =60W. This is the amount of power consumed in 1 hour. If the light bulb is used for 5 hours is it will consume 60x5=300W hours of power.

Taking an example of a clinic with maternity unit in rural Kenya. The facilities include the main clinic building with office and 2 surgery rooms. In addition, there is a small maternity building with ward and delivery room. Electrical appliances include, 10x10W light bulbs, 3x15W fans, a 65W laptop plus a 1kW vaccine fridge and a 2.7kW autoclave (130C and 0.2MPa).

The estimated electrical loads are as follows: The light bulbs have varying amounts of use, but the average is 4hrs per day 10x10x4 =400Wh/d This is the same for the fans 3x15x4=180Wh/d Laptop is used for medical record and is on 6hrs 65x6= 390Wh/d The fridge consumes power for about 1/3 of the time 1x.333x24=8kWh/d and the autoclave operate on 5-minute cycles and is used 4 times per day. 2.7x(5/60)x4 = 1kWh/d The daily load demand is 9.97kWh or 10kWh

Electricity can be generated by solar or wind technologies as well as diesel or a hybrid combination. In Kenya a stand-alone PV system was chosen. The components include PV modules, batteries, charge controller, inverter, wiring and mounting structure. There are conversion losses to be factored in to the design which may be around 20% of the generation. If lead acid batteries are used, they must not go below 50% charge and a degree of system autonomy should be allowed for bad weather when solar generation is limited, such as 3 days.

Solar estimates for the site show 6hrs of sunlight per day is available in the worst month. The demand is 10kWh and with the conversion losses 10x100/80=12.5kWh generation needed. The PV array must be 12.5/6hrs = 2.08kW round to 2.1kW The battery storage required will be 12.5 x 3 days = 37.5kWh With only 50% discharge 37.5x100/50 = 75kWh Using 24V batteries 75/24 = 3.125kA or 3,125Ah

A simple design sheet has been produced by Dr Matt Little helps further illustrate this. The figures presented are only estimates for Kenya, and a proper site-specific design model must be carried out. A simple tool PV GIS can be used to give a first scope for a project. There are numerous modelling software solutions available. HOMER is widely used and quite suitable but does have licencing costs. Interestingly solar powered vaccine fridges running on DC and solar thermal autoclaves are also possible options for rural medical centres.