PhotoVoltaic System Sizing

© ARJ 2002

• This is not a How-To presentation.

• It is a What and Why presentation.

Why?

1. People who run out of power become VERY unhappy!

2. People who pay for excess power they never use also tend to be unhappy.

What ?

do you need to size a system.

Basic Information

• Geographic location (including Altitude).

• Accurate Load Requirements.

• Grid-Connected or Stand Alone.

• Utility Backup or Days of Autonomy.

Geographic Location

• Solar Resource Available.• Temperature.• Seasonal Variation.• Array Tilt Angle.

Most data in the US is based on 10 - 30 year averages compiled by NREL etc. International data varies, but is generally available from the Meteonorm Database.

Obstructions & Shading

• If even a portion of a solar module is shaded the entire string will suffer GREATLY reduced output. It is therefore VITAL to understand how mush the array will be shaded and when.

• A Solar Pathfinder is an excellent tool to measure a locations shading.

• Eye-balling shading is NOT a reliable way to estimate shading!

Solar Pathfinder

Load Requirements

• DC Loads and time of Day / Season.

• AC Loads and time of Day / Season.

• Backup Loads (AC) for Grid-Tie systems.

Watt Hours (Wh)

• One way to consider a load is in term of Watt Hours —— (how many Watts for how many hours ?) This is how the information is presented on a utility bill.

• Example a 100W lamp running for 3½ Hours a day consumes:

100W 3.5 h = 350 Wh

• 1 kWh = 1000 Wh

Amp Hours (Ah)

• Some people prefer to specify loads in Amp Hours (especially in stand-alone systems).

• It is easy to convert between Ah and WhWatts = Volts Amps

Wh = Volts A h• Remember there may be several different

voltages in a system.

350Wh at 120V = 350/120 Ah = 2.92Ah

Load Profiles

• The load profile is often one of the hardest things to quantify. Especially in stand-alone residential systems. Customers almost ALWAYS underestimate the loads they use.

• Phantom Loads (small loads that are ALWAYS on, e.g. Clock on VCR – Microwave etc. , anything with a remote control) can have a BIG impact on the total System.

Stand-Alone

Load Sheet

Stand-Alone

Appliance Qty Volts AC/DC Run W H /Day D /Week Surge X Phantom W <Wh/Day>Refrigerator 1 120 AC 165 16 7 3 0 2640Flourescent Lamp 3 120 AC 16 4 7 1.2 0 192Incandescent Lamps 2 120 AC 4 12 7 1 0 96Computer 1 120 AC 15 2 7 1 0.1 32Chargers 3 120 AC 0 0 7 1.1 0.1 2Vacuum Cleaner 1 120 AC 840 0.5 1 2 0 60Tools 1 120 AC 1200 0.5 2 2 0 171

AC 3194

DC/DC 48V -> 12V 1 48 DC 0 0 7 1 24Motion Lamp 2 12 DC 25 0.1 7 1 29Outside Flourescent 2 12 DC 36 1 2 0 21

DC 74

Minimum Inverter Size (kW) 2.28 Ave. kWh/Day 3.27Total Surge Capacity (kW) 4.66 kWh/Week 22.87

Stand-Alone

Sizing is based on Load Requirements

• Loads can be simple and predictable: e.g.– Lighting; Remote monitoring; water pumping

etc. (things that don’t rely on people)

• Complex and difficult to predict: e.g.– Village power plant, remote homes etc. (things

that people interact with)

Grid-Tied Backup

Uninterruptible Power Supplies (UPS)

Sizing is based on Backup-Load and Economics

• Battery capacity and inverter size is determined on required load size and backup time.

• PV size is based on the economics of the Grid-Tie. —— Remember that the batteries can be recharged from the grid.

Grid-Tied Backup

Grid-Tied

Why do Grid-Tie?

• Public Relations

• Electrical Savings - Economics– Time of use rates– Peak-Shaving (Use less or sell power at peak use /cost times)

– Incentives (tax breaks, net metering, power selling)

– Economics of new-construction with current Energy codes (High efficiency buildings)

• Environmental Statement

Grid-Tied

Sizing based on Financial Optimization

• Area available for PV. (Tilt may not be an option)

• Installation Costs.

• Utility Rate Structures.

• Incentives.

• Integration into new building.

Grid-Tied

Sizing Process

Steps

1. Find insolation and temperature data for site.2. Optimize tilt or tracking considering shadowing. 3. Specify loads and load profile.4. Determine system losses (e.g. wiring, efficiencies).5. Specify days of Autonomy.6. Minimum 1.1 Array to Load (including losses) ratio.7. Battery state of charge always > 80%.8. Calculate PV & Batteries required to meet design.

*Stand-Alone

Example

• In Phoenix on average there are 5.5hrs of ‘good’ sunlight a day.

• A Kyocera KC-120 nominally produces 120W .

• However, the temperature is higher than the test temperature, 106W might be a better number.

• So on average we produce 583Wh /day. (106W 5.5h)

• If the inverter/cabling/battery efficiency is 75%.

• We can run a 100W lamp for :(538Wh 0.75) / 100W = 4.04 hours

Software

• There are several software packages available to help in the sizing process:– Solar Design Pro– PVSyst– RetScreen– Nsol– Etc.

• Many of the modern sizing programs perform detailed financial evaluations.