Operating Wireless Gateways Off-Grid With Solar PV Electric


Shared on August 06, 2012 written by Keith McRae

The intelligent wireless gateway and router, often referred to as a wireless modem, is perfect for connecting where there is no wireline data available. Therefore it is logical to connect the wireless wide area network modem where there is no grid electric readily available as well. The solution is to harvest energy from the sun using solar photovoltaic cells. Wireless gateways use very small amounts of electricity, and they often run on DC power, the same direct current electric we can generate from solar PV arrays. USAT architects and assembles single panel systems designed to use a pole mounted panel, a NEMA enclosure, and a battery array to power a wireless modem and connected equipment.

It is important to consider what equipment will be attached along with the wireless modem/intelligent gateway. Consult with your USAT sales representative before selecting attached equipment as disparate voltages, native AC powered devices, and high draw equipment can force the solar assembly to be oversized and cost prohibitive. For example, if you need to attach an IP Pan-Tilt-Zoom camera to the off-grid system, USAT can suggest low current PTZ cameras that work well with solar assemblies. A similar example is with network switches. Switches designed for 12Vdc or 24Vdc or POE systems should be selected, and these should be rated for a broad operating temperature, for example -40C to +60C.

Calculating modem power demand is based on measuring the known power consumption characteristics of the gateway. These can be broken down into power demands for four states: Transmit, Receive, Idle, and Power Save. USAT examines data usage, keep alive pings, and other modem activity to calculate the daily consumption and a variance factor to create both the maximum demand and the average the modem will have. USAT has modeled modem power consumption characteristics for many makes of wireless equipment including the Sierra Wireless Airlink Raven, Sierra Wireless Airlink PinPoint, Sierra Wireless Airlink GX-400, Sierra Wireless Airlink GX-440, Encore Bandits, Cradlepoint COR and MBR, Digi Connect, Digi WR, and Digi Transport, CalAMP, Multitech and Red Lion Sixnet Bluetree. Wireless modules designed for Sprint's network varies in energy consumption from T-Mobile's and AT&T's network which varies in energy consumption from Verizon 's network. Likewise we need to explore whether the modem will be on GPRS, HSDPA, HSUPA, or 1x-RTT, or EVDO rev A or 0. Likewise 4G presents different module consumptions whether running LTE, or WiMAX, or one of the HSPA+ variants. Each device has unique power requirements, and these all vary based on the intended application. Let USAT size the system for you leveraging our engineering expertise specific to solar and wireless data applications.

The core elements of an off-grid solar photovoltaic system are as follows:

Solar PV Panel

The Solar photovoltaic panel harvests electricity from the sun. Quality and efficiency vary widely from manufacturer to manufacturer. Panels often use polycrystalline silicon, monocrystalline silicon, amorphous silicon, cadmium telluride, and copper indium (di)selenide/sulfide (CIGS). CIGS has the highest absorption coefficient of solar modules and is useful in harvesting energy in sub-optimal conditions such as on cloudy and overcast days and has high efficiency and carry a high cost per watt. Monocrystalline thin film solar cells are often more expensive than "multi"crystalline or polycrystalline based cells, but have a greater efficiency which is useful for wireless off-grid installations and provide a balanced price to performance.

While researching ratings be mindful of whether the wattage is rated as PTC or STC. PTC or PVUSA Test Conditions uses a known constant developed by NREL (National Renewable Energy Labs). STC, or Standard Test Conditions, uses a methodology which produces higher results. Generally PTC is held to be more reflective of real-world solar and climatic conditions.

Understand the efficiency of the panel. On a pole mounted system , an efficient panel should be used to ensure that the system can be powered by one or two panels.

The panel must have its performance characteristics available, and these should follow a know standard such as PTC mentioned above.

PV Panel Mount

For the panel installation pole mounts will house up to 140 Watt in a single panel for a compact pole mount installation. These can be installed so the pole has two panels with minimal shading impact on the lower panel. Larger panels may require different bracing for the pole and mount. Panels will install angled toward the equator. For North America, this array azimuth is 180 degrees, due south. You should have a clear 'access' to the sun in a southerly facing mount. Other compass headings are feasible with a corresponding loss in productivity. Their pitch or array tilt angle is based on the location they are used in and how the system is optimized.

Solar Charge Controller

These take the solar energy and provide it to charge the battery storage in the system as well as provide electric to the system equipment (modems, cameras, meters, sensors, etc).

It is important to determine how critical array yield is, what the operating temperature requirement is, as well as if the controller will used in a hazardous location (Class 1, Division 2, Groups A-D, such as needed for oil & gas industry application).

Two major charge controller technologies are MPPT (maximum Power Point Tracking)and PWM (Pulse Width Modulation) charge controllers.

MPPT charge controllers maximize the yield from a panel by harvesting from the peak point of the PV array. The MPPT then convert the power to a charging voltage for the battery and boosts the power when needed. USAT often specs MPPT charge controllers that have a 97% or a 99% efficiency rating and a very low self-consumption rate. These are very useful when the yield from a one or two panels needs to be maximized—when adding another solar panel is not a practical option.

MPPT charge controlled benefit from using numerous power points during shading, sunrise and sunset, from very fast sweeping of the entire solar i-v curve, and from the strongest production during low solar insolation times. These often have temperature sensors that can adjust the charging temperature of the battery based on ambient temperature inside the enclosure. Solar sizing can be completes in watts since the panel will be fully utilized.

PWM charge controllers connects the array directly to the battery being charged, therefore operating the array at the battery voltage. The controller should have a 4 stage battery charging capability to maintain optimal battery life and charge state.

Array sizing must be done in amps as less power is available for charging the battery.

USAT can also send over the Wireless WAN modem solar operation information, alerts, and state triggers when a smart charge controller is used. They can also communicate on Modbus and other standards.

Battery

Specify an Absorbed Glass Mat (AGM) gel type sealed, maintenance free battery designed for use in solar applications. These are non-gassing so enclosure venting becomes less critical to prevent gaseous buildup. These also are not susceptible to freezing, and have a greater lifespan when compared to wet batteries. These batteries can carry DOT, IATA, ICAO, and IMDG approvals so they may be transported by land, sea and air.

The battery should be rated for temperature extremes present in outdoor enclosure, and should carry a warranty that covers a published derate (decay) factor based on age that does not exceed 25%.

The battery can be monitored by the charge controller for optimizing charging voltage based on the ambient temperature.

Always collect the table of discharge rates for the battery chosen. Often times an extended discharge time should be chosen, and the temperature present at discharge should be taken into account when modeling solar panel array size requirements.

NEMA Enclosure

This should be a NEMA enclosure with an IP (Ingress Protection) rating sufficient to keep the enclosed equipment safe from the elements (search for IP rating in USAT's common questions for more info).

Glass polycarbonate is the preferred material for the NEMA enclosure. Unless in areas with high security or vandalism risks, glass polycarbonate enclosure are sufficiently durable and conduct less heat to the enclosed equipment.

The enclosure should also be vented. Venting does allow excess heat to be passively removed, but also facilitates pressure equalization in changing weather conditions. Venting can be accommodated while still maintaining the units IP rating by the use of labyrinthine vents.

Generally speaking active venting is not suggested. Active venting uses fans to force air out of the box. Fans have the disadvantage of drawing environmental elements into the box including dust, airborne particulates, humidity, and even water. Fans also draw more electricity and can fail over time. If they must be used, they should be thermostatically controlled fans and highly energy efficient.

USAT has designed NEMA enclosures for use with WWAN modems. These assemblies have backplanes, batteries and venting specifically designed for field wireless use.

Keywords: Solar M2M Connections, Solar PV Electric, Off-Grid Connections

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