Application Considerations

1) Sizing of a standalone PV system

A simplified procedure is given below (adapted from the book Planning and Installing Photovoltaic Systems by German Solar Energy Society, 2005).

(a) Calculate the energy consumption requirements of the loads
  • From this procedure, we get the daily average electricity consumption requirement Ec in kWh/day.
   
(b) Calculate the number of PV modules required
  • Obtain the peak power rating of a PV module, P
  • Obtain the monthly figures for the "daily mean horizontal solar irradiation", in kWh/m2/day. (A set of such figures is given in the Solar PV - Resource Potential section.) Let's call this factor A. The factor A can be interpreted as being equivalent to 1000W/m2 irradiance shining for A hours. Therefore in the equation given below, the unit of A is taken as hours.
  • Obtain the monthly figures for the "tilt correction factor". Let's call this factor B. Depending on the month, B could be greater than 1 or less than 1.)
  • The ideal mean energy yield per day Ei (in Wh per day) of the module can be calculated as follows: Ei = P x A x B
  • Next we multiply by a factor to take into account cable losses, conversion losses, mismatch losses, and deviation of cell temperature from the standard value (actually the amount of deviation differs from month to month). When these factors are multiplied together, we get an overall loss/correction factor of Lf (less than 1). The mean energy yield per day Eg is equal to Ei x Lf
  • Choose a suitable number of modules so that the total Eg of the PV system is greater than Ec, for each month in the year, with a good margin.
   
(c) Calculate the size of the battery bank required
  • If we allow for D number of continuous overcast days, then for a M-volt battery bank, the required Ah rating is equal to D x Ec (in Wh) / M plus a good margin.
   
(d) Choose the cable sizes according to normal electrical practices, and choose a suitable charge controller.

2) Sizing of a standalone wind/PV hybrid system

In locations where there is sufficient wind resource, such as in the remote areas, a wind/PV hybrid system can serve to reduce the size of the battery bank as compared to a PV-only system. When there is a long period of continuous overcast sky, very often the wind will be blowing and the small wind turbine will provide some power to charge up the battery bank. Therefore, a smaller battery bank will be required. (A paper by Yang, Lu and Burnett given in Solar PV - Literature section deals with this subject.)

Regarding the application considerations when installing a small wind turbine, refer to the Small Wind Turbines section of this website.

3) Planning and designing grid-connected PV system

A grid-connected PV system does not require battery bank for storage. Furthermore, the electrical loads will be supplied simultaneous by the electricity grid and by the PV system, therefore it is not necessary to match the PV system to the loads. Very often, the limiting factor is the availability of suitable spaces for mounting the PV panels.

Grid-connected PV systems vary in size from a few kW to hundreds of kW. Some key steps in planning and design of a grid-connected PV system are given below.

  • Select a suitable location for installing the PV panels or PV glass units (for building-integrated PV systems). Check shading from nearby structures or buildings. Check structural requirements of the location.
  • Note that when the PV panels are inclined to the horizontal, they should be separated by certain distance between consecutive rows (facing south usually) in order to prevent mutual shading effect.
  • Carry out a preliminary design for the PV system. For systems of a few kW, the inverter can be housed in a weather-proof cubicle. For systems of larger size, an equipment room may be necessary. Also select an approporate distribution board for connecting the AC output of the inverter to the electrical distribution system of the site. Consult the "Technical Guidelines on Grid Connection of Small-scale Renewable Energy Systems" for the major design considerations (safety, equipment protection, reliability, power quality) of grid connection.
  • Submit an application for grid connection to the power company. The power company will advise their requirements for the grid connection (e.g. the protection settings).
  • Carry out detailed design for the PV system.
  • Install and commission the system.
  • The power company will conduct inspection on the system to confirm that the system can meet their stated requirements.

Some recommendations:

  • Tilt the panels at an angle approximately equal to the lattitude. This will maximize the year-round energy yield and will help remove accumulated dirts by rain.
  • Choose PV panels of high efficiency. (Due to limited space in Hong Kong, try to squeeze as much power as possible from the available space.)
  • Use PV panel certified to IEC or equivalent standards.
  • Choose an inverter with high reliability (e.g. with high mean-time-between-failure figure), high efficiency, and proven operational record (e.g. ask the supplier to provide list of job references for local or overseas applications).
  • Design the control system to switch off the inverters during night time, to reduce the standby energy losses of the inverters.
  • Request the supplier to recommend necessary spare parts.
  • Request the supplier to provide maintenance and trouble-shooting instructions for the inverter.
  • Inspect the installed system regularly during operation. Because of the grid connection arrangement, there will be no sign of trouble (e.g. no lights out) when an inverter failures. Only regular inspection will identify the failure in output of the PV system.

The performance of the pilot BIPV system (with three sub-systems) at Wanchai Tower was monitored for a year by EMSD. The 12-month performance monitoring work was completed in March 2004. The This web page has hyperlinks which may transfer you to third-party website.summary report provides useful information on the performance of different BIPV arrangements in a typical commercial office building in Hong Kong.

4) Shading analysis

To assess the extent of shading resulting from nearby tall objects and structures, a shading analysis is performed. Select a point in the PV array, then obtain the elevation angle and azimuth angle of each shading object relative to that point. Using a sun path diagram (or using software), the extent of shading in a particular month can be assessed.

The extent of shading will affect the choice of the disposition and circuit arrangement (number of modules in a string and number of strings in the array) of the PV arrays.

5) Software for design of PV system

Analysis or design of a stand-alone or grid-connected or PV system can also be done using computer software.

The This web page has hyperlinks which may transfer you to third-party website.RETScreen International Clean Energy Project Analysis Software is a decision support tool developed with the contribution of experts from government, industry, and academia in Canada. The software, provided free-of-charge, can be used worldwide to evaluate the energy production and savings, life-cycle costs, emission reductions, financial viability and risk for various types of energy efficient and renewable energy technologies (RETs). The software also includes product, cost and climate databases, and a detailed online user manual.

Commercial PV system design tools utilizing time-step simulation can be used for simulation and yield calculations, and added with shading analysis function, etc. Examples are This web page has hyperlinks which may transfer you to third-party website.PVSYST developed by the University of Geneva and This web page has hyperlinks which may transfer you to third-party website.Solar Studio Suite. There are also many other software available. Users should study the capabilities of each piece of software to determine whether it can satisfy their needs.

 

   
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