Some energy assumptions (Dr Nigel Scott)
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Tuesday, 11 February 2014 14:04
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In this piece I outline the thinking we went through in order to get a feel for the capacity of a small home system. Our starting point was the net amount of energy needed to cook a meal, but this proved a bit tricky to find. Not surprisingly, most studies are interested in the fuel consumed (which is where savings can be made) rather than what goes into the pot. There are also wide variations depending on what you are cooking e.g. boiling beans for 4 hours is quite different to frying some meat and vegetables. Based on the literature there seems to be discussion around the figure of around 2 MJ/capita/day. Rural households in Africa typically comprise 4 to 5 members so, based on a household size of 5, the cooking energy requirement (transferred to the food) will be around 10 MJ/day.

The concept relies on using a simple electric hob plate, which is a technology that people are familiar with, and behaves in a similar way to current cooking appliances such as a jiko, or fuel efficient stove. However, a single 500W electric hob would take over 5 hours to deliver the energy required, which is hardly practical. Of course, it may be possible to use higher capacity hob(s), but in this case we have explored the use of the system by a smaller, 2 person household. A 500W hob could deliver the energy required (4 MJ/day) in just over 2 hours.

The battery is likely to be the most challenging component in the system. Delivering the energy required will take just over 90 Ah from a lead-acid battery. However, the energy will be delivered over a short time period (2 hours), which is much faster than normal – most batteries are rated to discharge energy over a 10 or 20 hour period (C10, C20 rating). The effective battery capacity will, therefore, be around 80% of typical rated capacity. Although deep discharge batteries can be discharged up to 80% of their charge, restricting the discharge to a nearer 50% will help preserve battery life , so the system would need a battery rated at around 220 Ah. The efficiency of battery charging depends on the discharge level - the further a battery is discharged, the more efficient the charging process is. With an average discharge level of 50%, we have estimated the charging efficiency to be around 70% (Most sources estimate lead-acid batteries to operate at around 80-90% efficiency, but this refers to charging from a full discharged state). In order to deliver 4 MJ of energy, the battery would require 5.7 MJ of energy from the solar panels (or from minigroid or from main grid - since we are now looking beynd solar home systems per se).

At an estimated mean daily peak sun hours of 6 hours for a typical sub-Saharan location, a solar capacity of just over 250 Wp would deliver the energy required by the batteries. This would require a panel area of around 1.7 m2, which seems quite practical for mounting on the roof or in the garden of a small household.

Last Updated on Tuesday, 11 February 2014 14:10