fig2_solar_web

Consider solar-powered remote monitoring systems

Oct. 10, 2006
It could be cost prohibitive to employ your local electric utility to string wires so you can drop a meter at a remote location. Instead, read how solar-powered remote monitoring systems can save you money and solve your problems.

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You’ll find it cost prohibitive to have your local electric utility string wires and drop a meter at some remote location just because you need to install instrumentation and metering there. Instead, it’s more practical to use a radio telemetry-based SCADA system when there’s no 120 VAC power available. For the purpose of illustration, let’s assume that you want to install a remote, loop-powered, differential-pressure transmitter.

You’ll need to design and build a solar-powered system (Figure 1). These low-voltage DC powered systems are convenient for remote sites, oilfields and residential applications. With the high costs of fuel and utilities, it makes sense to look at investing in solar power systems.

Figure 1: Your solar array taps into the nuclear fusion that powers life on Earth.

Solar locations vs. efficiency

The first question to ask is whether a solar system will be efficient at the location. Solar is universal and can work anywhere, but some locations are more suitable than others. In sizing a solar-power electric system, consider two factors. One is insolation values (sunlight level); the other is the electrical load. Irradiance is a measure of the sun’s power available at the earth’s surface and it averages about 1,000 watts per square meter.

Obviously, some parts of the earth receive a greater number of daily full sun hours than others. Many Web sites feature maps of the locations best suited to solar systems. For example, an Alaskan site will have short days and lots of cloud cover, snow and storms during the winter. You’ll have a unique set of problems keeping that solar power system working. You may want to consider wind generators instead.

Calculating the loads

Follow the model shown in Table 1 to tally the total weekly watt-hour demand at the remote site. In the example, the total is 2,187.36 w-h/wk and the system operates on 24 VDC. Dividing 2,187.36 by 24 gives a battery discharge rate of 91.14 amp-hours per week, or 13.02 amp-hours per day. I’d recommend rounding up and applying a 1.5 service factor, making the solar electric system 50% larger to help ride through a series of cloudy days. The next task is sizing a bank of batteries with a sufficient amp-hour rating.

Everything in the example is DC powered. The spread-spectrum radio uses less than one watt. The PLC has a discrete input card and an analog input card. Measure the solar array voltage and the battery voltage with two analog inputs. The discrete inputs can be used for an intruder-alert door switch and any other input you may need.

Panels and trackers

A photovoltaic solar panel can be thought of as a direct current generator powered by sunlight. A layer of boron diffused into the upper surface of a glass substrate produces a thin P-type silicon layer. This is covered by a layer of arsenic-doped N-type silicon wafer. The resulting PN junction acts as a permanent electric field. When photons of sufficient energy strike the solar cell, they knock electrons free and force them through an external battery or DC load. The panels are sized by voltage and wattage.

Solar panels perform as designed only when they are facing directly into the sun. A few hours later, a stationary solar panel won’t be getting direct sunlight and the output drops by 50% to 75%. You can’t reposition the solar panel every few hours at a remote site. Tracking the sun increases the output efficiency of a solar array, allowing it to produce more continuous energy relative to stationary solar panels.

The solution for this problem is a solar tracker (Figure 2). The model I use has a metal hub that rotates on a metal base, powered by a small DC motor. A 12-inch by 6-inch split solar panel with a shadow plate acts as the controller (Figure 3). The two panels are wired in opposition and one side nulls out the other when the sun faces the panels directly. Use lightweight aluminum Unistrut and pipe in lieu of steel hardware for the solar panel mounting on the tracking unit. This increases the life of the tracker because extraneous weight can wear out a small DC motor.

Figure 2: Figure 2. Sun tracker keeps the unit oriented directly into the sun.
Figure 3: Figure 3. Photocells on the sun tracker controller activate the motion.

This small solar controller keeps the larger solar array oriented toward the sun. As the sun’s position changes, one side of the controller moves into shadow, which produces enough power to drive the little DC gear motor that spins the base and repositions the solar array for direct sunlight. Later in the day, the solar tracker is facing west as the sun sets. The next morning, two small solar panels on the back side catch the sunrise and reorient the tracker to the east, when the split-panel controller takes over.

Charge controllers

Select a charge controller that measures:

  • How much power the solar array is producing
  • Battery voltage
  • Instantaneous load current
  • Whether the system is charging or not

Look for an optional remote display, which minimizes having to open the control panel. A fully automated unit can be left alone for months at a time. Lower-end models will work and they use LEDs. A digital display of the pertinent information is better and you won’t have to use a hand-held meter to check voltages the LEDs can’t display.

Battery selection

Battery capacity is measured in terms of amp-hours. Size the battery bank to accommodate three to four cloudy days with no solar charging. Although similar to ordinary car batteries, the batteries (Figure 4) used in a solar-electric system are specialized for the daily charge/discharge cycling they’ll need to endure.

Figure 4: You’ll need to have sufficient battery capacity for maintaining power during a string of cloudy days.

Solar applications use one of three battery types — gel batteries, sealed batteries or flooded batteries. The freeze-resistant qualities of gel batteries make them a good choice for unheated spaces. Sealed batteries are virtually maintenance-free. The flooded lead-acid batteries are usually the least expensive, but require adding distilled water occasionally to replenish water lost during the normal charging process. You could conceivably wire two 12-volt batteries in series to get 24 volts, or wire them in parallel for more storage capacity.

Inverter selection

Inverters are rated by continuous watts and surge watts. Continuous wattage is the total watts the inverter can support indefinitely. For example, a 4,000-W inverter can power a 4-kW load continuously until the batteries discharge. If the same inverter was rated at 7,000 surge watts, it can handle 7,000 W of momentarily inrush while loads such as motors get started.

Not all solar-power systems need an inverter. In the case of the example flow-metering station, we’re using only DC power to operate the whole station. If the system has an inverter, you’ll need to analyze your load requirements carefully.

Inverters are sized by wattage, so you can calculate the amperes using Ohm’s Law. Don’t assume anything or everything can be plugged into an inverter. Heating units, for example, which consume huge amounts of energy, aren’t designed for efficient consumption at low power.

The inverter’s output sine wave might not be as clean as the 60 Hz from your utility. Connect an oscilloscope to the 120 VAC output to compare the waveform with that from your power company, and be sure it’s compatible with your loads. Like anything else, you get what you pay for.

Maintenance issues

Let’s start at the top and work our way down. Easy maintenance access is a consideration when choosing the location for the solar array. The solar array’s vertical angle will need to be adjusted slightly for summer and winter months because the sun’s track changes relative to the solar array orientation.

The solar panels might need to cleaned once a month, depending on how much airborne dirt, snow or bird droppings they accumulate. The tracker hub assembly might require greasing about once a year. In some applications, ambient gases can corrode exposed metal parts.

The control panel enclosure should be able to withstand the environment, rain and wind. You might want to consider some form of lightning protection because the solar array will probably be the highest metal device in the area. Make sure the control panel has enough room to accommodate everything that has to go inside it. There’s nothing more inconvenient for a maintenance technician than having to try to work inside a control box that’s overflowing with components.

The control panel enclosure also can serve as the battery box, especially for no-maintenance sealed batteries that eliminate worry about battery gases, adding water or overcharging boil — over acid spills.

The general reliability of a solar-electric system makes it possible to avoid the cost of bringing utility lines to remote sites and having to pay another electric bill.

Being off the grid requires making adjustments in the use of electricity so as to operate within the limitations of the system’s design. This doesn’t necessarily imply doing without, but rather shifting to a more conscientious use of electricity. Much information and inexpensive solar equipment can be found on the Web. I’d enjoy hearing about your alternate energy adventures.

Tom Hamilton is operations manager at A-Tech Controls in Mansfield, Texas. Contact him at [email protected] and 800-760-3780.

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