Zunis turn to the Sun


Clay Bravo of the Hualapai Tribe

If you need to supply water beyond the reach of power lines, then solar power can help solve the problem. Photovoltaic powered pumps provide a welcome alternative to fuel-burning engines and hand pumps. Thousands of solar pumps are working throughout the world. They produce best during sunny weather, when the need for water is greatest.

How it works

Photovoltaic (PV) panels produce electricity from sunlight using silicon cells, with no moving parts. They have been mass-produced since 1979. They are reliable enough that most manufacturers give a ten-year or greater warranty, and a life expectancy of 20 years or more. They work well in cold or hot weather.

Solar water pumps are specially designed to utilize direct current (DC) electric power from PV panels. They must work during low light conditions at reduced power, without stalling or overheating. Low volume pumps use positive displacement (volumetric) mechanisms which seal water in cavities and force it upward. Lift capacity is maintained even while pumping slowly. These mechanisms include diaphragm, vane and piston pumps. These differ from conventional centrifugal pumps that need to spin fast to work efficiently. Centrifugal pumps are used where higher volumes are required.

A surface pump is one that is mounted at ground level. A submersible pump is one that is lowered into the water. Most deep wells use submersible pumps. A controller or current booster is an electronic device used with most solar pumps. It acts like an automatic transmission, helping the pumps start and not to stall in weak sunlight.

A solar tracker may be used to tilt the PV array as the sun moves across the sky. This increases daily energy gain by as much as 55 percent. With more hours of peak sun, a smaller pump and power system may be used, thus reducing overall cost. Tracking works best in clear sunny weather and in locations with horizon-to-horizon sunlight. It is less effective in cloudy climates and on short winter days, or where obstacles limit the number of hours of direct sun.

Storage is important. Three to ten days of storage may be required, depending on climate and water usage. Most systems use water storage rather than batteries, for simplicity and reliability. A level sensor can turn the pump off when the water tank fills, to prevent overflow. A similar control can turn the pump off if the water source is drawn too low.

Compared with windmills, solar water pumps are less expensive, and much easier to install and maintain. They provide a more consistent supply of water. They can be installed in valleys and wooded areas where wind exposure is poor. A PV array may be placed some distance away from the pump itself, up to several hundred feet away.

What it is used for

Livestock Watering: Cattle and sheep ranchers on the Ute, Navajo, Zuni, and Hualapai Reservations - as well as many others in North America, Mexico, and Australia are enthusiastic solar pump users. Their water sources are scattered over vast rangeland where power lines are few, and costs of transport and maintenance are high. Some ranchers use solar pumps to distribute water through several miles of pipelines. Others use portable systems, moving them from one water source to another. Solar pumps are also used to protect streams and ponds by pumping to troughs away from fragile aquatic ecosystems.

Irrigation: Solar pumps are used on small farms, orchards, vineyards and gardens. It is most economical to pump PV array-direct (without battery), store water in a tank, and distribute it by gravity flow. Where pressurizing is required, storage batteries stabilize the voltage for consistent flow and distribution, and may eliminate the need for a storage tank.

Aquaculture: Solar pumps are used for aeration, or to provide circulation in ponds to prevent eutrophication and icing.

Domestic Water: Solar pumps are used for private homes, villages, medical clinics, etc. A water pump can be powered by its own PV array, or by a main system that powers lights and appliances. In a combined system, more configurations are possible. An elevated storage tank may be used, or a second pump called a booster pump can provide water pressure. Or, the main battery system can provide storage instead of a tank. Rain catchment can supplement solar pumping, when sunshine is scarce. To design a system, it helps to view the whole picture, and consider all the resources. Solar water pumps are often part of low-power remote water purification or water treatment systems, using ultraviolet light or chlorine to make water safe to drink. (See News Briefs for more information about ultraviolet (uv) water purification).

Thinking Small

There are no limits to how large solar pumps can be built. But they tend to be most competitive in small installations where combustion engines are least economical. The smallest solar pumps require less than 150 watts, and can lift water from depths exceeding 200 Feet (65 m) at 1.5 gallons (5.7 liters) per minute. You may be surprised by the performance of such a small system. In a 10-hour sunny day it can lift 900 gallons (3,400 liters). That's enough to supply several families, 30 head of cattle, or 40 fruit trees!

Slow solar pumping lets us utilize low-yield water sources. It also reduces the cost of long pipelines, since small-sized pipe may be used. The length of piping has little bearing on the energy required to pump, so water can be pushed over great distances at low cost. Small solar pumps may be installed without heavy equipment or special skills.

The most effective way to minimize the cost of solar water pumping is to minimize water demand through conservation. Drip irrigation, for example, may reduce consumption to less than half that of sprinkler irrigation. In homes, low flow toilets can reduce total domestic use by half. Water efficiency is a primary consideration in solar pumping economics.

A Careful Design Approach

PV panels are expensive, so we must size our system carefully. It is like fitting a suit of clothes: you need all the measurements. Here is a guide to the data that you will need to determine feasibility, to design a system, or to request a quote from a supplier.

Step 1. DESCRIBE WATER REQUIREMENTS

Typical daily water requirements
People: 10 to 100 gallons per person per day, depending on lifestyle and conservation measures
Cattle: 10-30 gallons per day in dry weather
Small Animals: 1/4 gallon per day per 25 lb. body weight
Poultry: 6 to 12 gallons per hundred birds per day
Young Trees: 15 gallons per day in dry weather

- These figures will vary with local conditions -
Amount needed: (e.g.,100 gallon per day average)
Uses: (home? irrigation? livestock?)
Variation in water requirements: (e.g.,50 gallons per day in January, 300 gallons per day in July)
Will the solar pump be the only source of water? If not, what percentage of the supply will it provide?

Step 2. DESCRIBE THE WATER SOURCE

If it is a deep well:
Well depth
Depth to water
Recovery (production) rate (gallons/minute or day)
Additional vertical lift required to an elevated tank or up a hill
Horizontal distance from the source to the point of storage or delivery

If the water is less than 20 feet below the surface:
Describe the source (e.g.,pond, spring)
Vertical lift required to the tank location
Horizontal distance from the source to the point of storage or delivery

Step 3. DESCRIBE ANY EXISTING EQUIPMENT

Is a working pump already in place? (Describe)
A storage tank?
A pipeline? (State length and diameter)
Is there commercial power available at the site? How far away? (Solar pumping may not be cost-competitive with existing utility service.)

Step 4. DESCRIBE THE SOLAR ENERGY AVAILABLE AT THE SITE

How many sun-hours are available per day? Use a solar map showing average insolation, such as the one on this page. If you need to pump most in the winter time, use a map that shows winter sun hours. This can be derived from databases and maps available from a variety of sources.

Note whether there are any obstructions to direct sunlight at the site.

Note if there are any local variations from the regional climate, (i.e.coastal fog).

Step 5. DETERMINE WHAT KIND OF PUMP YOU NEED

This is based on the nature of the water source. Submersible pumps are suited both to deep well (bore hole) and to surface water sources. Surface pumps can only draw water from about 20 feet (3m) below ground level, but they can push it far uphill. They are less expensive than submersibles and a greater variety is available.

Step 6. DETERMINE THE FLOW RATE REQUIRED

Here is the equation, in the simplest terms:
Gallons Per Day / Available Peak Sun Hours per day=Gallons per Hour
(for example, 600 gallons per day/10 peak sun hours=60 gallons/hour)

Step 7. IDENTIFY APPROPRIATE PUMP AND SOLAR ARRAY

Refer to the performance charts published by solar pump manufacturers who produce the type of pump that is appropriate. They will specify the size and configuration (voltage) of solar array necessary to run the pump.

-Windy Dankoff and Chris Greacen

Contact Chris Greacen at NAREEP or Windy Dankoff at Dankoff Solar Pumps and Solar Products, 505-473-3800