Photovoltaic System Sizing-Part 1

Photovoltaic System Sizing

Media Clip: Solar Pathfinder

When describing PV systems, it is logical to follow the energy flow from the array side to the loads. However, when sizing a PV system, it is necessary to consider the energy demand before considering the supply. Therefore, PV-system sizing, particularly for stand-alone PV-systems, starts at the load side and proceeds backward to the array. The objective is to first determine the

requirements of the system loads and then to determine the size of the inverter, battery bank and array that are needed to meet the requirements. Since there are many possible PV-system configurations, each with different modes of operation and priorities, the approach and methodology used to size these systems may differ. There may also be a difference in the sizing tolerance; some systems may need to be sized more carefully than others.

Sizing Interactive Systems

Interactive systems require relatively simple calculations and allow the widest variance in component sizing. Since interactive systems operate in parallel with utility service, sizing is not critical because failure of the PV system to produce energy does not affect operation of electrical loads. Additional energy can be imported from the utility at any time.

Sizing interactive systems begins with the specifications of a PV module chosen for the system. Module ratings at Standard Test Conditions (STC) are used to calculate the total expected array DC power output per peak sun hour. This is then de-rated for various losses and inefficiencies in the system, which includes the following:

• Guaranteed module output that is less than 100% • Array operating temperature

• Inverter power conversion efficiency • Inverter MPPT efficiency

Refer to PDF and Excel WorkSheets for Interactive System Sizing

Interactive System Sizing

Enter the following values into the Excel worksheet:

PV-Module Rated DC Power Output: 185 W Manufacturer Power Guarantee: 0.90

Number of Modules in Array: 16

Array Guaranteed Power Output: _______ W Array Avg Operating Temperature: 50 °C

Temperature Coefficient for Power: -0.004 /°C

Temperature-Corrected Array Power Output: _______ W Array Wiring and Mismatch Losses: 0.03

Net Array Power Output: _______ W

Inverter Maximum DC Power Rating: 2500 W Inverter Power Conversion Efficiency: 0.92 Inverter MPPT Efficiency: 1 .00

Inverter Maximum AC Power Output: ________ W Average Daily Insolation: 5.1 Peak Solar Hours/day Average Daily Energy Production: ________ kWh/day

Interactive-system sizing is very flexible because the utility can supply extra energy to the system loads and receive excess energy from the PV system.

The result is a final AC power output that is substantially lower but realistically accounts for expected real-world conditions. To determine the expected energy production per day, the final AC power output is multiplied by the insolation in peak sun hours per day. For example, if the calculated AC power output is 2140W per peak sun hour and the average annual insolation is 5.1 peak sun hours then the average energy production is expected to be 10.9 kWh/day. The size of an interactive system is primarily limited by the space available for an array and the owner's budget. However, financial incentive requirements, net metering limits, and existing electrical infrastructure may also influence system size decisions. Even if short-term periods of high insolation or low demand result in excess electricity, it is not wasted because it can be sold back to the utility for credit against subsequent utility bills.

The only exception is a system that is so large that it maintains a net energy export over several months or more. Because many utilities will not carry the credits for more than one year and /or will credit exported electricity at lower wholesale rates, it is not recommended to size an interactive system larger than needed for average annual on-site load requirements.

Sizing Stand-Alone Systems

Stand-alone PV systems are designed to power specific on-site loads, so the size of these systems is directly proportional to the load requirements. If the system is too small, there will be losses in load availability and system reliability. If the system is too large, excess energy will be unutilized and wasted. Therefore, sizing of stand-alone systems requires a fine balance between energy supply and demand.

Because of this necessary balance, sizing stand-alone systems requires more analysis and calculations than are required for interactive systems. Most of these calculations build upon one another as the analyses proceed. Moreover, sizing stand-alone systems is an iterative process. That is, if the final calculations indicate that the components are improperly sized, the starting values must be changed and the calculation process repeated until the system output matches the load requirements.

Stand-alone PV systems must be carefully designed to meet all the load requirements without excessive over-sizing.

Refer to Stand Alone System Sizing, Load Analysis pdf

Sizing Bimodal Systems

Bimodal systems normally operate as interactive systems, but can operate as stand-alone systems during utility outages. Therefore, the bimodal systems are typically sized according to the stand-alone methodology. However, a significant difference between bimodal systems and true stand-alone systems is that

bimodal systems typically supply only a few select critical loads while in stand-alone mode.

The load analysis used for sizing a bimodal system should include calculations for only these critical loads, which are needed during a utility outage. The rest of the subsequent calculations for inverter rating, battery-bank energy storage, and array output are identical to those for stand-alone systems. Similarly, the battery-bank sizing allows for the desired back-up period without grid power from a few hours to several days.

The stand-alone sizing methodology determines the minimum size of a bimodal system. However, since excess energy produced during the normal interactive mode can be exported to the utility grid for credit; there is no penalty for

oversizing a bimodal system, at least within the guidelines of sizing conventional utility-interactive systems.

Sizing Hybrid Systems

Hybrid systems are stand-alone, which can't rely on the utility as a source of electricity. These systems must be able to completely and reliably supply power to their on-site loads. However, there is more than one source of energy, such as a combination of a PV array and an engine generator or wind turbine. The presence of multiple power sources means that the array and battery bank can be smaller while maintaining load availability, especially if one source can provide power on demand, such as an engine-generator.

The array and battery bank for a PV array and engine generator hybrid system are sized similarly to those for a stand-alone system, with three differences. First, the array is sized to supply only a portion of the total load requirement. For example, the hybrid system may be designed such that 80% of the load demand is supplied by the array and 20% is supplied by the generator on an average day. Second, the sizing calculations do not need to use the worst-case load-to-insolation months for sizing, since the engine generator can be called upon to provide additional power as needed. Average load and insolation values may be used. Finally, battery banks can be sized for a shorter autonomy period (typically only 1 or 2 days) than for PV-only stand-alone systems, also because the generator power is available on demand.

PV array to engine generator output ratios range from 90%:10% or to 40%:60%. The optimal ratio is determined by performing sizing calculations using several different ratios and choosing the system that best fits other requirements (such as available array space) and has the lowest expected life-cycle costs. Other factors, such as minimizing the average run time of noisy engines, may also influence sizing.

Combination PV array and engine genera tor hybrid systems are relatively easy to size because the generator output is completely dependent on demand. Other energy-source combinations, however, such as a PV array and a wind turbine or a PV array and a micro-hydroelectric generator, are much more difficult to design and size adequately because their outputs are less predictable.

SIZING CALCULATIONS

Sizing PV systems for stand-alone operation involves four sets of calculations. First, a load analysis determines the electrical load requirements. Then, monthly load requirements are compared to the local insolation data to determine the critical design month. Next, the battery bank is sized to be able to

independently supply the loads for a certain length of time, such as if cloudy weather reduces array output. Finally, the PV array is sized to fully charge the battery bank under the critical conditions.

A hybrid PV and engine generator system utilizes array energy better (wastes less energy) than PV-only systems because more of the available energy is utilized. In some cases, these systems may also cost less overall than PV-only or engine generator-only stand alone systems sized far the same load

requirements

Load Analysis

Analyzing the electrical loads is the first and most important step in PV-system sizing. The energy consumption dictates the amount of electricity that must be produced.

All existing and potential future loads must be considered. Underestimating loads will result in a system that is too small and can't operate the loads with the desired reliability. However, overestimating the load will result in a system that is larger and more expensive than necessary. Comprehensive yet

conservative load estimates will ensure that the system is adequately sized. A detailed load analysis completed during the site survey lists each load, its power demand, and daily energy consumption.

Refer to separate Stand Alone System Load Analysis.doc sheet.

for AC loads goes through the inverter, resulting in losses that must be accounted for separately.

Power Demand

Peak-power information is usually found on appliance nameplates or in

manufacturer's literature. When this information is not available, peak power demand can be estimated by multiplying the maximum current by the operating voltage, though this is less accurate for reactive loads. Measurements, meter readings, or electric bills may also be used to help establish existing load requirements.

The peak power demands are then summed. The total power demand is considered when determining the required inverter AC-power output ratio. While it is not likely that every load would be ON at the same time, it is recommended to size the inverter with extra capacity.

Energy Consumption

Electrical energy consumption is based on the power demand over time. Loads rarely operate continuously, so each load's operating time must be determined. This is the total number of hours per day that the load is operating.

The operating time for loads that cycle on and off automatically is typically determined from the duty cycle. Duty cycle is the percent age of time a load is operating. For example, a duty cycle of 40% means that a load is operating 40% of the time, or 9.6 hr/day (40% x 24 hr/day = 9.6 hr/day). Even loads that are plugged in all the time, such as refrigerators and air conditioners, have a variable power requirement based on duty cycle.

User-operated loads are turned on and off manually. Determining the operating time for these loads is simple if they cycle only once per day. However, if loads are switched on and off several times per day, a metering device is probably the easiest method of determining the operating time.

The daily energy consumption for each load is determined by the load's power demand multiplied by the daily operating time. For example, a 60 W light bulb that is on for 4 hr/day consumes 240 Wh of energy (60W x 4hr=240Wh).

Some loads may not be used every day. In these cases, the average daily operating time is calculated by dividing the total operating time over a longer period by the number of days in the period. For example, a washing machine that operates for 2 hr/wk has an equivalent operating time of 0.29 hr/day (2 hr/wk - 7 days/wk = 0.29 hr/day).

The AC energy consumption and DC energy consumption values are totaled separately. These values are used to determine the total amount of DC energy the system must produce.

Operating Time

Load operating-time data is also used to size the battery bank. For consistent loads that operate for specific periods, calculating the daily operating time is very simple. For example, if the loads are night time lighting fixtures that operate for 6 hr each night, the daily operating time is 6 hr.

Most often, however, there are multiple loads to consider that each operate for various lengths of time. The battery-bank discharge rate will then change as various loads turn ON and OFF during the day.

If the system includes both AC and DC loads, the AC load energy requirement must be first be converted to equivalent DC energy. This is done by dividing each AC energy consumption amount by the inverter efficiency.

Inverter Selection

If the system includes AC loads, an inverter must be selected. Several factors must be considered when selecting the inverter. First, the inverter must have a maximum continuous power output rating at least as great as the largest single AC load. A slightly oversized inverter is usually recommended to account for potential future load additions. The inverter must also be able to supply surge currents to motor loads, such as pumps or compressors, while powering other system loads.

Inverter voltage output is another consideration. Most stand-alone inverters produce either 120 V single-phase output or 120/240 V split-phase output.

Some higher-power inverters for commercial or industrial electrical systems output three-phase power. Alternatively, certain inverters can be stacked and operated in parallel for split-phase output.

The inverter DC-input voltage must also correspond with either the array voltage (for interactive systems) or the battery-bank voltage (for stand-alone systems).

Inverter Efficiency

Inverters are not 100% efficient. Some power is lost in the process of converting DC energy to AC energy. Therefore, more DC energy is required to produce a certain amount of AC energy. Both the AC and DC energy requirements from the load analysis are used to determine how much total DC energy will be required. The total amount of DC energy required by the loads is calculated using the following formula:

ESDC = required daily System DC electrical energy (in Wh/day)

EAC = AC energy consumed by loads (in Wh/day)

= inverter efficiency

EDC = DC energy consumed by loads (in Wh/day).

For example, if a load analysis determines that a system requires 800 Wh/day for the AC loads and 200 Wh/day for the DC loads and the inverter efficiency is 90%, what is the daily DC electrical energy required by the system?

ESDC = ( EAC / ) + EDC

ESDC = ( 800/0.9 ) + 200

ESDC = 1089 Wh/day

Inverter efficiency is typically between 80% and 95%. Also, an inverter's

over most of its power range. Manufacturer's specifications will typically include efficiency ranges. For sizing calculations, the average efficiency for the expected operating power range should be used.

Critical Design Analysis

A stand-alone system must produce enough electricity to meet load

requirements during any month. Therefore, systems are sized for the worst-case scenario of high load and low insolation. A critical design analysis compares these two factors throughout a year, and the data for the worst case is used to size the array. The critical design ratio is the ratio of electrical energy demand to average insolation during a period. The load data comes from the load analysis, which is usually performed for each month. The insolation data is available from the solar radiation data sets. The ratio is calculated for each month.

Critical Design Month

The critical design month is the month with the highest critical design ratio. This is the worst-case scenario, and the associated load and insolation data are used to size the rest of the system.

Refer to Stand Alone System Sizing; Critical Design Analysis pdf

If the loads are constant over the entire year, the critical design month is the month with the lowest insolation on the array surface. For most locations in the Northern Hemisphere, this is a winter month, either December or January. However, when the load requirements vary from month to month, the critical design month must take into account both the loads and the available

insolation. Because of these two factors, the critical design month may turn out to be any month of the year.

Sizing for the critical design month typically results in excess energy at other times of the year, if this excess is significant, the system designer may want to

consider adding diversion loads or changing to a different system configuration, such as a hybrid system, that better matches the available electrical energy to the loads.

Array Orientation

Since array orientation has a significant effect on receivable solar radiation, array orientation must also be accounted for in a critical design analysis. If the mounting surface restricts the array to only one possible orientation, then the analysis is conducted to determine the critical design factors for that

orientation.

However, if multiple orientations are possible, separate analyses are performed for each orientation. A critical design month can be identified for each of the array orientations, since the receivable solar radiation will be different for each. Of the resulting critical design months, the one with the smallest design ratio is the best choice. This orientation minimizes the required array size, while still accounting for the worst-case load-to- insolation situation.

The orientations most commonly used in a critical design analysis are tilt angles equal to the latitude, latitude+ 15°, and latitude- 15°, each at an azimuth of due south.

The greater array tilt angle maximizes the received solar energy in winter months, and the smaller array tilt angle maximizes the received solar energy in summer months. Insolation data for these orientations is available in the solar radiation data set for the nearest location.

If tracking systems are to be used, receivable insolation data for the various tracking modes can be used instead of fixed array orientations.

The critical design ratio is calculated for each month for each array orientation or tracking mode. The highest critical design ratio for each orientation

corresponds to the critical design month for that orientation. When multiple orientations are considered, the lowest critical design ratio of the resulting critical design months corresponds to the optimal array orientation (of the

orientations analyzed). The insolation and load requirements for this month and array orientation are used in subsequent sizing calculations to design the array and battery system.

DC-System Voltage

The DC-system voltage is established by the bank voltage in battery-based systems. This voltage dictates the operating voltage and ratings for all other connected components, including DC loads, charge controllers, inverters, and (for battery-based systems) the array.

DC voltage in battery-based systems is critically important. The DC voltage for battery-based PV systems is usually an integer multiple of 12 V, usually 12 V, 24 V, or 48 V.

DC loads, charge controllers, and inverters that operate at these voltages are commonly available. The selection of the battery-bank voltage affects system currents. For example, a 1200W system operating at 12 V draws 100A (1200W- l2V= 100A). The same 1200Wsystem draws only 50A at 24V,or 25A at 48 V. Lower current reduces the required sizes of conductors, overcurrent protection devices, disconnects, charge controllers, and other equipment. Also, since voltage drop and power losses are smaller at lower currents, higher-voltage systems are generally more efficient.

As a rule of thumb, stand-alone systems up to 1 kW use a minimum 12 V battery-bank voltage, which limits DC currents to less than 84A. Similarly,

battery voltages of at least 24 V are used for systems up to 2 kW, and at least 48 V for systems up to 5 kW. Very large stand alone systems may use battery

voltages of 120V, though battery banks over 48 V involve additional code requirements and safety measures.

System Availability

The size of a system in relation to the loads determines its system availability. System availability is the percentage of time over an average year that a stand-alone PV system meets the system load requirements. For example, 98% system availability means that a system is able to meet the energy demand about 98% of the time. This means that for 2% of the year, the system can't meet the load requirements.

No energy-producing system can achieve 100% availability, because of unpredictable events that affect system output. Days or weeks of

below-average insolation, such as unusually cloudy weather, will reduce short-term system availability. System availability can also vary between years due to long-term weather patterns. Component failures and lack of maintenance also

contribute to system downtime and reduce system availability.

System availability is determined by insolation and autonomy. Accurate

estimates of system availability require software to evaluate energy flow in the system on an hour-by-hour basis, but rough estimates are adequate for most PV applications. For a desired system availability, the designer chooses the

appropriate length of autonomy.

Autonomy is the amount of time a fully charged battery system can supply

power to system loads without further charging. Autonomy is expressed in days. Most stand-alone systems are sized for a system availability of about 95%

(about 3 to 5 days of autonomy) for noncritical applications or 99% or greater (about 6 to 10 days or more) for critical applications.

However, each percentage-point increase in system availability is increasingly more expensive for larger battery banks and arrays, which is impractical from an economic stand point for all but the most critical applications.

Sizing of stand-alone systems must achieve an acceptable balance between system availability and cost goals for a given application. The solar resource for a location also affects the increasing costs of availability.