Page 183 - Applied Photovoltaics

9.1.6 Petrol generators
When compared with diesel generators, petrol generators:
x produce lower emissions of greenhouse gases
x are lighter and more transportable
x are cheaper for a given output rating
x can be converted to LPG
x require more maintenance as they operate at higher speeds (3000 rpm versus
1500 rpm for diesel)
x are less efficient
x are noisier
x have smaller fuel tanks, necessitating more frequent refuelling
x have much shorter lives (typically 1000 hours versus 10,000 hours for diesel).
9.1.7 Hybrid system design

General considerations
In view of the disadvantages cited above, a diesel-only system has severe limitations
in operating efficiency. The addition of batteries alone greatly enhances system
efficiency, although it necessitates the inclusion of an inverter and control circuitry.
For a diesel-battery or photovoltaic-diesel-battery system, batteries with appropriate
characteristics must be selected. For instance, many batteries commonly used in
photovoltaic systems have strictly limited charging/discharging rates, which could
lead to their destruction if used in a diesel system with less than three days of storage.
Other types of battery are only for ‘float’ applications where they remain almost fully
charged most of the time.

Batteries used in RAPS systems with diesel generators must be of the deep-cycle
type, but do not necessarily need low self-discharge rates. Fork lift batteries are
probably a cheap, well suited type of battery for such systems. However, most
photovoltaic system suppliers now offer specially designed ‘solar’ batteries and these
should be used wherever possible.

Inverters are expensive and where possible should be downsized from the peak loads
by having periods of peak demand coinciding with periods of diesel or petrol
generator usage.
For a fixed array size exactly matching the load, the loss-of-load-probability (LOLP)
is given in Fig. 9.5 as a function of days of storage. By definition, an array size that
exactly matches the load is one where the total annual photovoltaic generation exactly
matches the total load consumption.

170