The Solar Power System for the Experimental House 0

Solar photovoltaic panels

Solar photovoltaic panels

Solar Power – Photovoltaics

The process of converting solar energy to electrical energy is called photovoltaic power generation. The photovoltaic cell (or solar cell) was invented in the early 1950s, with the increase in semiconductor technology.  Photovoltaic panels are made from silicon – either a single crystal, amorphous crystals, or a thin film. Considerable research is being done to find other materials, such as organic materials, that would be suitable. When sunlight strikes the surface of the semiconductor, it transfers energy to some of the electrons so that they are no longer bound to the nucleus and are free to move through the material. By connecting wires to the panel you can use this current of free electrons to do work on an outside circuit.

Designing the Solar Power Sytem

Installing the solar panels

Installing the solar panels

In northern latitudes there is a large difference between the length of the potential charging day on the summer solstice and the winter solstice. Designing a solar power system for such conditions usually involves a compromise that aims to meet the load requirements for three quarters of the year and uses approximately one hundred hours of generator backup to supplement the solar and wind charging for the one quarter of the year with the shortest days. This is a compromise designed to make the system more cost efficient, since designing a power system to meet winter needs would result in excess energy in the summer that would often be wasted.

The load requirements and the average expected solar radiation at the proposed site are used to calculate the appropriate size of the solar array. Solar radiation data are available from NASA for the grid square from 51 to 52 degrees latitude and 106 to 107 degrees longitude. The proposed building site is situated near the northwest corner of this grid square.

A solar cell receives the maximum amount of power if it is tilted at an angle perpendicular to the sun’s rays. To achieve this condition for all times of the day and days of the year would require a two-axis tracking device which would add considerable expense and complexity to the system. Most residential systems have the array at a fixed angle or at two tilt angles that are adjusted for the season.

This method produces a reasonable compromise to maximize the solar gain and help shed snow in the winter.  The table below shows the size of solar array required for a two tilt angle system.

Array Size to Meet a 4.8 kWh/day Load Requirement with two tilt angles, at Average Solar Radiation and Minimum Solar Radiation

Month Daily Ave. Radiation at tilt 51º (kWh/sq.m/day) Daily Ave. Radiation at tilt 66º (kWh/sq.m/day) Required Array Size (kW) Daily Min. Radiation at tilt 51º (kWh/sq.m/day) Daily Min. Radiation at tilt 66º (kWh/sq.m/day) Required Array Size (kW)
January 2.92 3.1 2.06 2.66 2.82 2.26
February 4.22 4.33 1.47 4.05 4.15 1.54
March 5.11 4.97 1.25 4.68 4.54 1.41
April 5.19 4.74 1.23 4.96 4.53 1.41
May 5.23 4.56 1.22 4.67 4.08 1.56
June 4.84 4.2 1.32 4.37 3.82 1.67
July 4.38 3.85 1.46 3.86 3.41 1.87
August 4.26 3.81 1.50 3.77 3.38 1.89
September 3.99 3.77 1.60 3.31 3.11 2.05
October 3.5 3.5 1.82 2.73 2.71 2.36
November 3.02 3.16 2.02 2.29 2.38 2.68
December 2.58 2.76 2.31 2.41 2.57 2.48
Annual 4.1 3.89 1.56 3.64 3.45 1.85

The array size is calculated by dividing the daily load requirement by the daily average radiation and then multiplying the result by 1.33 to compensate for losses and inefficiencies in the system.

Solar Array on the Experimental House

Solar Array on the Experimental House

Since the power system design includes both a solar panel array and a wind generator, the solar array only needs to supply about one third to one half of the load requirements.

The system was designed with a 920 Watt solar array consisting of eight 115 Watt Evergreen solar panels. This product was selected because their innovative string ribbon manufacturing process provided excellent efficiency rating at a lower cost than other manufacturers because it eliminates the wastage of saw cuts.


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