Solar Irrigation: The Economics of Large Versus Small Systems

©2003 Windy Dankoff

We receive many questions from around the world, regarding the cost of solar irrigation for farms that now use water pumps powered by diesel or gasoline (petrol) engines. Solar pumps are not yet being produced in large sizes (greater than 5HP). This article explains why, and suggests an alternative for larger pumping requirements.

SMALL solar pumps (less than 5HP) can be very competitive in relation to SMALL diesel or gasoline engines. LARGE solar pumps, in relation to LARGE diesel engines, are MUCH LESS competitive. Here are the reasons:

1. A larger diesel engine is more fuel-efficient and requires less maintenance PER HORSEPOWER than a small one. This is called "economy of scale". A comparable economy of scale also applies to the economic feasibility of extending the electrical mains (electric power lines, utility grid).
2. Solar-electric pump technology can benefit only a slightly from economy of scale. (The solar array is made of of the same small photovoltaic (PV) modules, whether it is sized for 0.5KW or 50 KW.)
3. The cost of a solar pump (final cost to the user) is approximately $10,000 per KW. NOTES: 1 kilowatt (KW) produces approximately 1 true HP. Diesel or gasoline (petrol) engines use a false rating method in which 1 true HP = approximately 2.5 HP engine rating. Example: A 50HP diesel engine at full capacity produces approximately 20 true HP, equivalent to a 20HP electric motor and equivalent to peak output of a 20KW PV array. The end user's cost of a 20KW solar system, delivered and installed at 1999 retail prices) can be as high as $200,000.
4. Today's market for small solar pumps (less than 2HP) is far greater than the market for larger ones. Therefore, solar pump manufacturers concentrate on products in the 1/6 to 2 HP range. There are very few larger systems in existence, and they are not being produced regularly at the time of this writing (late 1999).

The best way to utilize solar pumping for a large farm may be to use a multitude of small pumps, and to use them intensively (to repay the investment). For optimum utilization, Conergy suggests that you consider the advantages of PORTABLE solar pumps. For example, a 1KW (1HP) system uses a PV array that is only about 100 square feet (10 sq. meters) in size, and weights about 350 pounds (160 kg.). The system can be mounted on a trailer or cart to be moved easily. It can also be rotated in order to track the sun for maximum output at the beginning, middle and end of the day.

How much water will a 1KW, 1HP solar pump move? As an example, with a vertical lift of 15 feet (5m) it can pump about 140 gallons (530 liters) per minute in full sun. In one day, it will pump about 40,000-70,000 gallons (150-250 cubic meters). The flow is inversely proportional to the lift, so if the lift is 30 feet (10m), it will move half as much water, etc. (This example is approximate. Check actual product specifications to determine the performance of a pump for your situation.)