Planning, Installation and Maintenance of Solar Home System
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 dependent 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. RETScreen and NASA give the value of the yearly horizontal solar irradiation of any point just by giving latitude and longitude of the place.
► Go to the NASA portal.
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 irradiation 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 irradiation 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 irradiation 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.
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
Typical Meteorological Year (TMY)
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.
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. ► Solar irradiation data from NASA and RETScreen
To size a PV system it is important to know the average power demand of its users. The power demand depends on the energy consumption of the appliances (loads) connected to the system. The more energy is consumed and the more appliances are to be connected, the "bigger" the system has to be. A higher system output can be reached by using modules with a higher nominal output or by connecting more modules to a system. Generally, it is possible to expand an installed system if the users' power demand grows. PV systems only produce direct current (DC). DC is commonly found in many low-voltage applications such as lights, radio, TV or mobile phone chargers. If alternating current (AC) appliances shall be powered, an inverter must be used to convert the DC power produced by the solar cells. Inverters are expensive, but AC systems have the advantage that AC appliances cost less than DC appliances and are readily available.
Solar Home System Sizing Example
This sizing example is based on estimation, just to give you an idea about the size and capacity of a solar home system.
|Appliance||No. of appliances||Consumption in watt||Hours of operation/day||Wh/day|
|TV 14" B&W||1||15||4||60|
|Mobile phone charger||1||1||4||4|
|Total energy consumed by load (Wh/day)||196|
Estimation of Peak Sun Hours
PSH (peak sun hours) = number of hours per day when solar irradiance averages 1 kW/m² = solar insolation in kilowatt-hours per square meter per day For example: 5.05 in Managua, Nicaragua (worst month: December)
PV System Output
Wh = Wp x PSH
@ Standard Test Conditions (25°C and 1000 W/m2)
Wh = Wp x PSH x PR PR (performance ratio) = relationship between the actual energy output and theoretically possible output of a PV system, includes battery efficiency, wire loss, temperature compensation, etc. Typical value: PR = 55% Example: Wh = Wp x 5.05 x 0.55 = 196 Wp = 70.57 ⇒ Consider a 75 Wp PV module
Wh = n x energy load (Wh/day) / DOD n = no. of days of autonomy = days without sunshine DOD (depth of discharge) = energy withdraw from a battery, as a percentage of full capacity
Example: Wh = 4 x 196 / 0.70 = 1120 Wh Wh = Ah capacity x U U=terminal voltage Ah = Wh / U
Assumption: Battery terminal voltage: 12 V Ah= 1120 Wh/ 12V Ah = 93.33 Ah Choose a 12 V, 100 Ah battery
A Solar Home System has to be installed by a trained technician who knows how to handle its different parts. Example of solar training materials.
In order to guarantee the correct functioning of the system it is important to
inspect it on a regular basis. Wiring and contacts have to be free from corrosion, the modules have to be intact and clean and the mounting structure has to be fastened tightly. Dirt and dust on the modules lead to a lower module performance. The modules can be cleaned by the users of a PV system according to site conditions. If it rains periodically and air pollution is low, cleaning is required less frequently. The maintenance requirements for batteries varies significantly depending on the battery design and application. Maintenance considerations may include cleaning of cases, cables and terminals, tightening terminals, water additions, and performance checks. Performance checks may include specific gravity recordings, conductance readings, temperature measurements, cell voltage readings, or even a capacity test. Battery voltage and current readings during charging can aid in determining whether the battery charge controller is operating properly. If applicable, auxiliary systems such as ventilation, fire extinguishers and safety equipment may need to be inspected periodically. Generally speaking, flooded lead-antimony batteries require the most maintenance in terms of water additions and cleaning. Sealed lead-acid batteries including gelled and AGM types remain relatively clean during operation and do not require water additions. Battery manufacturers often provide maintenance recommendations for the use of their battery.
Solar Technical Training Manuals
Solar Photovoltaic Systems: a Technical Training Manual / UNESCO Publishing 2003 / Herbert Wade.
In his book Herbert Wade comprehensively covers all aspects of Solar Energy for rural applications. He illustrative compares electricity with water in order to make Solar Energy and how it works easy to understand. This is a basic Training Manual for Solar Trainers, Teachers, Installers and everyone who works in this industry.
- PVCDROM: http://pvcdrom.pveducation.org/SUNLIGHT/RADDATA.HTM
- PVCDROM: http://pvcdrom.pveducation.org/SUNLIGHT/RADDATA.HTMhttp://pvcdrom.pveducation.org/SUNLIGHT/TMY.HTM
- PVCDROM: http://pvcdrom.pveducation.org/SUNLIGHT/AVG.HTM
- James P. Dunlop, Florida Solar Energy Center for Sandia National Laboratories: Batteries and Charge Control in Stand-Alone Photovoltaic Systems. Fundamentals and Application, 1997.