Planning, Installation and Maintenance of Solar Home System

Planning

Photovoltaic Systems

Measurement of solar radiation

In PV system design it is essential to know the amount of sunlight available at a particular location at a given time. The two common methods which characterize solar radiation are the solar radiance (or radiation) and solar insolation. The solar radiance is an instantaneous power density in units of kW/m². The solar radiance varies throughout the day from 0 kW/m² at night to a maximum of about 1 kW/m². The solar radiance is strongly dependant on location and local weather. Solar radiance measurements consist of global and/or direct radiation measurements taken periodically throughout the day. The measurements are taken using either a pyranometer (measuring global radiation) and/or a pyrheliometer (measuring direct radiation). In well established locations, this data has been collected for more than twenty years.

An alternative method of measuring solar radiation, which is less accurate but also less expensive, is using a sunshine recorder. These sunshine recorders (also known as Campbell-Stokes recorders), measure the number of hours in the day during which the sunshine is above a certain level (typically 200 mW/cm²). Data collected in this way can be used to determine the solar insolation by comparing the measured number of sunshine hours to those based on calculations and including several correction factors.

A final method to estimate solar insolation is cloud cover data taken from existing satellite images.

While solar irradiance is most commonly measured, a more common form of radiation data used in system design is the solar insolation. The solar insolation is the total amount of solar energy received at a particular location during a specified time period, often in units of kWh/(m² day). While the units of solar insolation and solar irradiance are both a power density (for solar insolation the "hours" in the numerator are a time measurement as is the "day" in the denominator), solar insolation is quite different than the solar irradiance as the solar insolation is the instantaneous solar irradiance averaged over a given time period. Solar insolation data is commonly used for simple PV system design while solar radiance is used in more complicated PV system performance which calculates the system performance at each point in the day. Solar insolation can also be expressed in units of MJ/m² per year and other units and conversions are given in the units page.

Solar radiation for a particular location can be given in several ways including:

• Typical mean year data for a particular location
• Average daily, monthly or yearly solar insolation for a given location
• Global isoflux contours either for a full year, a quarter year or a particular month
• Sunshine hours data
• Solar Insolation Based on Satellite Cloud-Cover Data
• Calculations of Solar Radiation

Source: PVCDROM

 

Typical meteorological Year (TMY) Data

The most common data for describing the local solar climate is through what is called Typical Meteorological Year data (TMY). To determine TMY data, various meteorological measurements are made at hourly intervals over a number of years to build up a picture of the local climate. A simple average of the yearly data underestimates the amount of variability, so the month that is most representative of the location is selected. For each month, the average radiation over the whole measurement period is determined, together with the average radiation in each month during the measurement period. The data for the month that has the average radiation most closely equal to the monthly average over the whole measurement period is then chosen as the TMY data for that month. This process is then repeated for each month in the year. The months are added together to give a full year of hourly samples.

There is no strict standard for TMY data so the user must adjust the data to suit the application. Considerable care must be taken with sample periods. TMY data is used for a wide variety of meteorological applications and therefore a large amount of data is usually irrelevant for photovoltaic applications. Of the parameters given, usually only the time and irradiation figures are used. However, more advanced models also use the temperature and wind speed.

Source: PVCDROM

Average Solar Insolation

Although TMY data is commonly used for PV system simulation, the average daily solar radiation at a location in a given month is often sufficient for a basic system analysis. This data may be presented either as measured on the horizontal or measured with the measuring surface perpendicular to the solar radiation (corresponding to a PV system which tracks the sun). In either case, an additional angular dependence to account for the tilt of the module will need to be incorporated in order to determine the amount of solar radiation available to a PV module.

Source: PVCDROM