Anatomy of an off-grid stand-alone “all-DC” PV system
As the world's telecommunication networks are extended and upgraded, greater focus is being placed on the provision of rural communication services — where site access is often difficult, and connection to a central electricity grid is seldom possible. To power the telecom installations, such as repeater stations located in remote areas, with a high level of solar radiation levels year-round — and where the cost to extend the power lines is prohibitive — photovoltaic (PV) technology is the most economical option.
Direct current (DC) power systems serve as lifelines in today's telecommunication networks. With the availability of energy-efficient auxiliary system equipment operating on DC voltage, there is no need for an inverter, thus making an “all-DC” PV system possible. This article outlines the main features of a typical remote-area, stand-alone, all-DC PV-powered system with reference to design features, installation, and sizing.
Basic system components
Figure 1 (click here to see Fig. 1) shows the block diagram of a typical off-grid stand-alone PV system. A solar PV array, battery, and charge controller are the three primary components of the PV system. The solar array generates DC power for the load and charges the battery, which serves as the energy storage device that powers the load when there is no output from the array. The charge controller regulates the output of the PV array and ensures proper charging of the battery, thus protecting it from abuse. A portable gen-set is required to power the telecom equipment in case of no power output from the PV system.
Array — A PV system starts at its simplest level with a cell that is arranged into a group to form a module. A solar module typically consists of 36 cells and provides a nominal voltage of 12V. Solar modules vary in size from 1W to a few hundred watts. Many modules are connected to one another to form a panel (sub-array). The size of the sub-array is dictated by the weight and size that can be effectively handled at the site. The sub-arrays are then installed and wired to each other to form the array for the required voltage and current output.
Batteries — For off-grid, stand-alone solar power systems, batteries provide the backup power needed when sunlight is insufficient, such as during periods of heavy cloud cover and during the night. The battery bank should have sufficient ampere-hour (Ah) capacity to supply load during the longest expected period with no contribution from the PV array — it has to be a full-size battery bank with a good reserve for five or more days (120 hr).
PV batteries generally have to discharge a smaller current to the load for a longer period (such as all night), while being charged during the day. The batteries must be suitable to withstand the heavy daily cycling required for this application. One cycle is defined as a full charge followed by discharge. It is usually considered to be discharging from 100% to 20%, which means a “depth of discharge” of 80%. After discharging to 20% capacity, the PV system should be capable of recharging the battery to 100% capacity within 30 days (typical) with the defined peak sun hours per day.
Deep-cycle, lead-acid batteries are the type most commonly used in a solar system. Deep cycle batteries are designed to discharge most of their stored energy without damage and then re-charge hundreds or thousands of times while still maintaining long life. Deep-cycle batteries produce less current than a shallow-cycle battery but can produce that amount of current for a much longer period of time. It is common practice for a system to be designed with deep-cycle batteries even though the daily or average discharging amounts to a relatively shallow depth of discharge.
Vented (flooded) lead-acid (VLA) batteries are commonly used in PV systems, because they are more durable in cycling operations than other battery types. VLA batteries provide a long reliable service life of 20 yr (minimum), if deployed in a temperature-controlled environment, operated at the manufacturer specified charge voltage, and looked after appropriately by performing routine maintenance. VLA batteries are relatively inexpensive and readily available.
Telecom battery banks operating at 48VDC are assembled by combining 24 cells of 2V/cell (jar) batteries connected in series, each battery cell of capacity from 100 Ah to 6,000 Ah. In a single string battery system, an open circuit failure in a single cell will affect the entire string with catastrophic loss of function. Therefore, for reliability's sake, the bank is built by splitting the required battery Ah capacity between two or more parallel strings. In this arrangement, if a single cell fails to open, this doesn't result in loss of complete functionality.
Charge controller/regulator — Output from the solar modules varies considerably, depending on the prevailing sunlight and temperature conditions. Therefore, it's required to regulate its output by wiring it to the charge controller before it goes on to the battery bank and loads. Once the batteries are fully charged, the charge controller doesn't let current from the PV modules flow into them. Similarly, once the batteries have been drained to a certain predetermined level, the charge controller will not allow more current to be drained from the batteries until they have been recharged. The charge controller also eliminates any reverse current flow from the battery back to the solar modules at night. For reliability of system operation, the controller is arranged in a redundant configuration, providing 100% redundancy. The controller is also built with alarm features to indicate abnormal conditions (e.g., array failure, battery low-/high-voltage, circuit breaker trip, equipment overload, module anti-theft alarm, etc.) with provision for annunciating the alarms to a central station.
System sizing
Defining the system load is the first step to PV system design. The size and cost of a PV system is proportional to the energy consumed by the load. Reducing the electrical load by selecting energy-efficient equipment will result in a corresponding reduction in PV system cost. For an optimum design, it's important that designers have a thorough knowledge of the load profile with their duty cycle.
Ah capacity of the battery bank is computed first by multiplying the number of hours of battery reserve by the total load current. For a battery bank with two parallel strings, the Ah rating of the battery cell is found as one-half the calculated Ah — for three parallel strings as one-third of the calculated Ah, and so on. For the required cell Ah rating, the battery cell is selected from the vendor catalog at the design discharge hours for the desired “end of discharge” voltage.
The solar array must be capable of producing power to supply the load current for 24 hr/day and to supply the battery charging current for the daily peak sun hours at the site location. (Different parts of the world have varying daily peak sun hours. Refer to a solar insolation map for any particular location to determine the peak sun hours per day.) Daily output power of the solar module, selected from the vendor catalog, is found by multiplying the module output current by the daily peak sun hours. The number of required solar modules is found by dividing the required total daily power by the module's daily output power. The charge controller is sized to handle the sum of total load current and battery charging current. Typical Equipment Sizing Calculations for a Telecom PV System provides the sizing calculation for a typical telecom PV system with equipment housed in shelter.
Array installation
In the northern hemisphere, the sun tracks along a southerly route due to the earth's inclination to the sun. Therefore, it's important to orient the solar array to the south for locations in this area (the reverse is true for locations in the southern hemisphere). The array should face true south at a tilt angle equal to site latitude with respect to horizontal position. Tilt from horizontal is required to achieve a better angle at the sun and help keep the solar modules clean by shedding rain or snow. Tilt angles of latitude ±15° will enable increased energy production toward winter or summer, respectively. Fixed arrays are often favored for systems that operate without maintenance personnel on-site.
PV modules are very sensitive to shading. Once a solar cell or a portion of a cell is shaded, it becomes a load and draws power instead of producing it. Thus, PV modules should never be shaded by nearby trees or structures. Temperature at the back of the modules can rise to 80°C if they are poorly ventilated. Installing the solar array in an area with natural ventilation will improve system performance and extend its life. The mounting option must allow for safe maintenance and possible replacement of individual modules.
Shelter design and installation
Remote sites are invariably harsh environments with temperature and humidity varying widely throughout the year. High winds and dust may also be present. Although the PV modules and associated mounting components must be installed outdoors, all other system components and telecom panels should be installed inside an air-conditioned, dust-proof and water-proof shelter designed to withstand the wind loads in the area. With the availability of energy-efficient, DC-powered air-conditioning (A/C) units and luminaires, an all-DC PV system is a common reality in remote communication sites. Figure 2 (click here to see Fig. 2) shows the one-line diagram of a typical stand-alone PV system powering telecom and shelter loads.
The PV shelter is built with two rooms — one for housing the battery bank and the other for housing telecom equipment panels and other PV system components. The shelter shell is designed and manufactured as monolithic panels for more strength and structural integrity. The shelter is installed on a reinforced concrete foundation designed to support the dead load of the shelter and for the expected wind speeds/gusts.
As the access to equipment inside the shelter is restricted to authorized personnel, battery systems are installed on open racks. The battery room is air-conditioned to provide a temperature-controlled environment, which translates into a long, reliable life for the batteries. To avoid the accumulation of hydrogen given off at the negative plates of the battery due to electrolysis of water in the electrolyte, exhaust fans are provided to effect the required air changes. The equipment room (housing the telecom panels and other PV system components) is also air-conditioned.
The PV shelter is installed directly under the solar array in such a manner that the array provides complete shading through the summer months, which reduces the heat gain into the shelter. Thus, the A/C power requirement is reduced considerably, helping to reduce the total PV system load and optimize the sizes of battery and solar array components. Figure 3 (click here to see Fig. 3) shows the side elevation of solar array with PV shelter located underneath.
Final thoughts
PV system design requires you to use good engineering practices, which includes complying with standards, employing experienced PV designers, and using components appropriately. Using service technicians experienced with PV systems to install and repair the system provides a major boost in system reliability. For off-grid stand-alone applications, an all-DC PV system with features as noted above has proven to be reliable.
Pandian is a project manager for Petro-Infrastructure Engineering Consultants Co. (Pi-Consult) in Al-Khobar, Saudi Arabia. He can be reached at pandian.2s@pi-con.com or pandian.2s@hotmail.com.
Telecom equipment = 520W
PV charge controller = 15W
DC A/C in equipment room = 360W @ 50% duty cycle (720W @ 100% duty cycle)
DC A/C in battery room = 216W @ 30% duty cycle (720W @ 100% duty cycle)
Exhaust fan in battery room = 30W
PV shelter lighting = 60W
Misc. loads (fire alarm system, etc.) = 25W
Total load in watts = 1,226W
Total load in current = Total load watts ÷ system voltage = 1,226 ÷ 48 = 25.54A
Unadjusted battery Ah = load current × battery reserve hours = 25.54A × 120 hr = 3,065 Ah
Adjusted battery Ah = unadjusted Ah × adjustment factor
Adjustment factor = age compensation × temperature compensation × depth of discharge (battery depth of discharge 80%) = 1.25 × 1.1 × 1.25 (see Note 1) = 1.72
Adjusted battery Ah = 3,065 × 1.72 = 5,272 Ah
Battery bank with three parallel strings, each battery string rating = 5,272 Ah ÷ 3 = 1,757 Ah
Battery cell of Type XXX deep-cycle batteries, from Manufacturer AAA, having 1,800 Ah @ 120-hr discharge rate to 1.85V/cell is selected for each battery string. Number of battery cells required for each 48V string is 24. Hence, the total number of battery cells in the bank is 3 × 24 = 72, with each cell rated 1,800 Ah, 2VDC.
Battery bank rating = 1,800 Ah × 3 = 5,400 Ah
Battery charging current to charge from 20% to 100% (i.e., 80% depth of discharge) = (battery Ah to be recharged × inefficiency factor) ÷ recharge hours = [(5,400 Ah × 0.8) × 1.15] ÷ 120 hr = 41.4A
Daily battery charging power = battery charging current × daily peak solar hours = 41.4A × 4 hr (see Note 2) = 165.6 Ah
Daily load power = load current X load duration (hours) = 25.54A × 24 hr = 613 Ah
Unadjusted total daily power = daily battery charging power + daily load power = 165.6 Ah + 613 Ah = 778.6 Ah
Adjusted total daily power = unadjusted total daily power × adjustment factor
Adjustment factor = PV array ageing X dirt accumulation × future growth = 1.1 × 1.2 × 1.1 (see Note 1) = 1.452
Thus, adjusted total daily power = 778.6Ah × 1.452 = 1,131 Ah
Daily peak output of PV module (for selected 12VDC PV module) = current @ maximum power × daily peak solar hours = 4.8A × 4 hr (see Note 2) = 19.2 Ah
No. of parallel PV modules = adjusted total daily power ÷ daily peak output of PV module = 1,131 Ah ÷ 19.2 Ah = 58.9, rounded up to 60
Number of series connected 12V PV modules for 48VDC system = 48 ÷ 12 = 4 modules
Total number of PV modules required 60 × 4 = 240 modules
Nominal power output of the selected PV module is 85W @ 12VDC
Total power rating of PV array is 85W × 240 = 20,400W (i.e., 20.4kW @ 48VDC)
Ampere rating = (load current + battery charging current) × service factor = (25.54A + 41.4A) × 1.1 = 66.68A × 1.1 = 73.35A
Current rating of charge controller @ 48VDC must be 75A or more.
Notes:
Adjustment factors for de-rating purpose are as defined in design practice/industry standards.
Daily peak solar hours is taken as 4 hr for the location under consideration.