For most stream applications, grid power is not available, and a remote power system must be utilized. Remote power may be provided by a battery–only system or by a battery combined with a generator.
Duration and budget for the project may narrow these choices. Budgets should include the cost of purchase, operation, and maintenance, as well as labor required to maintain all system components. Other factors to consider are
A first step in designing an off–grid power system is to calculate the "power budget," or total load for the system. This is accomplished by summing the wattage ratings listed on each system component. Total load in amperage is then calculated using the formula: amps = watts/volts, where "volts" is the voltage of the power source (e.g., 24 V). A typical power budget for an instream monitoring system is 0.8–3 amps.
Two high–quality 12–V rechargeable batteries can provide the least expensive method of powering the transceiver and related equipment with the needed 24–V DC power. Batteries need to be connected in series: the number used and their amp hour rating will in part determine the amount of power available, and thus the operating time before recharging is needed.
Batteries must be monitored and should not be deeply discharged, as this will cause damage and significantly shorten their life span. Acceptable discharge rates to ensure longevity are specified by manufacturers. For battery–only sites, a minimum of four 80–125 amp hour "deep cycle" 12–V batteries is needed, with four additional batteries that can be recharging for exchange with the installed set (see photo).
Three types of batteries are generally available: flooded lead acid, absorbed glass mat (AGM), and gel cell. Gel cell batteries are least appropriate for battery–only systems as they do not tolerate deep discharges to the same extent as the other two types.
Flooded lead acid batteries will tolerate repeated deep discharges and recharges and will therefore last longest. However, AGM batteries are safer for handling, since they do not contain liquid acid and do not vent gas during normal operation.
A battery bank with four 12–volt deep cycle batteries (80–125 amps) can last 4–7 days, depending on power budget, ambient temperature, and battery condition.
Batteries that are drawn down frequently tend to last only 1–2 years because they tend to be discharged to a point that damage occurs. This is commonly seen in studies that rely on exchanging batteries.
In terms of the purchase price, a battery–only system may seem most economical; however, there are other expenses to consider, such as the personnel and transportation to move batteries between the stream site and recharging location.
If a generator of any type is to be used, then additional factors must be considered:
In remote areas where propane cannot be commercially delivered, we mount a 379–L propane tank on a trailer, which enables transport of the tank for filling. During the period in which the interrogation system is without the TEG, it operates directly on the batteries.
We have successfully installed a number of thermoelectric generators (TEG) in our projects to maintain electrical charge on a set of two 12–V batteries in addition to powering the transceiver and ancillary equipment. The model used most frequently is a 54–watt 24–V DC unit, which consumes about 5.6 L of propane per day.
The most important advantage of the TEG over traditional generators is that it produces electrically quiet DC power (i.e., does not generate any EMI). The TEG also lacks moving parts, creates little noise, is reliable and requires little maintenance or secondary containment of the fuel. Overall, the TEG has a relatively small environmental footprint.