Part 1: Solar Decoded

In this, the first of our two part series, we are looking at Solar Panels both thermal and photovoltaic. In particular, looking into the payback times involved when installing a solar panel. Both of the articles are based on papers I have submitted recently for my master in Renewable technology through the University of Ulster and while some of the calculations used in the paper are beyond the scope of this article the results should speak for themselves.

With the recent revision of the building regulations requiring 10kWh/m² of the energy used in a new home come from a renewable source, solar panels are the most obvious choice for most home owners. This along with the retrofit market puts Solar Panels the No 1 choice for renewable energy. There is a vast array of panels on the market and choosing the most efficient and most suitable panels for your location can be a daunting exercise. In this article we will look at two different solar panels, a flate plate and an evacuated tube collector and determine which one is best suited to the Irish Climate and which one will give the best payback time.

Measured Data

For this exercise measured data for a solar HW heating system is used on two different panels. The data used is from the EUROSOL^[1] sheet which gives solar radiation data from 6 locations around Europe. This data can be downloaded here and is interesting to note how the solar radiation data compares from location to location. The two solar panel systems used have an installed area of 4m² with the first panel a flat panel collector made by Gasokol, an Austrian manufacturer, and the second system is an evacuated tube system by Sunda, a Chinese manufacturer of solar panels.

Optimum Angle

If, for example, we move the panels to southern Italy the figures change dramatically with the flat plate collector performing better with an efficiency of 62.9% compared to the evacuated tube system at 56.3%

The orientation and angle of inclination of the panels will of course make an impact on the performance of the system. The obvious orientation is due south and any deviation from this will have an effect on the performance of the system and in turn impact the payback times involved. Less obvious is the angle of inclination which is affected by the location of the dwelling which needs to be taken into account for optimization. For this sample case we are using a fictional dwelling located approx 50 miles from Cork city on the south coast of Ireland.

Using the sunbird jrc online calculator^[2] the optimum angle is calculated at 36 degrees for our location. With a typical rooftop angle of 40 degrees this should be close enough to receive the maximum radiation at this location. If required the angle could be adjusted on the roof to obtain the optimum angle but as we shall see it may be more useful to have the collectors tilted towards a more optimum angle to receive the winter sun as these months have the largest deficit for measured solar radiation.

Since the demand for hot water can usually be met quite easily during the summer months as there is a surplus of energy obtained from the panel to supply the required household hot water needs, hence the requirement for a heat dumps to be installed into the system, but the winter months have the largest solar radiation deficit and it may be more suitable to set up the angle of inclination toward a more optimal winter setting. We will look into this later in the article and see if adjusting the angle has any real impact on the performance of the panels.

Estimating Hot Water Usage

We can calculate the estimated hot water used in a typical house by the number and quantity of water used for each shower, bath and basin of hot water used each week. Although each house uses different volumes of hot water we will say that a typical household uses 400 litres of hot water a week which equates to 17,799 litres of hot water per year. In this case we are assuming that the hot water is heated to between 60 – 70^o C and is used at approximately 40^o C after passing through a mixing valve which would be the common setup used in most Irish houses.

Wasted hot water must also be factored in when we are trying to calculate the payback times for each system. A typical hot water cylinder holds 200 litres of water and we know that for this example there is 342 litres of hot water used every week. If we assume that 50% of this hot water is wasted due to dilution with the cold water and losses to the immediate environment this means that we need to heat 4 X 200 litres to meet the demand of 342 litres of hot water used each week. In this case we are assuming that the dwelling in not a high energy efficient house which would reflect the vast majority of house throughout the country.

Using these figures we can calculate the energy required to meet the annual hot water demand of the house. This give us a figure of 1253.14 kWh/yr for 400 ltires of hot water but when we factor in the waste water we need to double this requirement of energy use to 2506 kWh/yr.

Efficiency

If we now look at our two solar collectors, each with 4 meters squared of installed collector area we can look at the efficiencies calculated using the “radiation data” from the EUROSOL spreadsheet for our location. Comparing the energy extracted from solar insolation and the energy required to heat water over the year gives a distorted view of what portion of the dwellings requirements can be met, as there are months when the insolation is lower than required and, conversely, months when it is higher. It needs to be looked at on a month by month basis.

The efficiency of the flat plate collector comes out at 31.5% while the evacuated tube gives us an efficiency of 45.8% based on our location. What is interesting to note is how the efficiencies change depending on what location the panels are used. If, for example, we move the panels the southern Italy the figures change dramatically with the flat plate collector performing better with an efficiency of 62.9% compared to the evacuated tube system at 56.3%. The superior efficiency of the flat plate collector when located in southern Italy is down largely to the increase in solar radiation and the higher ambient temperature.

Now that we know the efficiency of each collector under similar conditions we can estimate the total output for each system. Again, assuming that the dwelling requires 2506 kWh/yr to meet the demand of the domestic hot water the flat plate collector can supply 58.3% while the evacuated tube 84.7% of the dwellings hot water demand.

Inclination

These figures are based on the annual solar radiation values at our location which give a distorted view as we should be looking at the energy supplied month by month. This is because during the summer months we are going to have a surplus of energy and during the winter months a deficit of energy. When this is taken into account the efficiencies drop to 57.4% for the flat plate collector and 73.7% for the evacuated tube system.

As we mentioned earlier we could adjust the inclination of the panels to capture more of the winter sun when there is a deficit of energy but when we run the calculations we find that there is only a small increase in the annual hot water demand satisfied when the angle is changed from the recommended 36 degrees to a more favourable winter angle of 60 degrees.

The graph below shows how the surplus summer energy can be reduced by adjusting the angle of inclination, thus allowing the collector to be exposed to a higher level of solar radiation during the winter months, when most additional energy is required for hot water use.

In saying that, the differences are small when the collector’s efficiencies are included. The table below shows the comparison of a 36 degree and 60 degree collector when compared and shows a 2% difference in annual required delivery energy but in opposite directions for the flat plate and evacuated tube systems. This is due to the flat plate systems loosing more in the summer months that it gains in the winter and the annual
deficit rises from 1068kWh to 1121kWh. The evacuated tube system performs better as the energy surplus in the summer is distributed to the winter months which drive the annual deficit from 659 kWh to 616kWh. It would not seem necessary to incorporate the additional costs imposing a 60 degree inclination on a residential installation. Saying that it may be cost effective to optimise the angle of inclination for a large commercial installation.

Cost

If we now look at the capital outlay for each system, including the grants that are available and calculate the saving based on the current price of natural gas we get a payback time of 29.2 years for the flat plate collector and 23.8 years for the evacuated tubes.

These payback times should be viewed as the maximum payback times, particularly when we consider the current government policy in relation to carbon taxes. Also when when we consider the current price of energy is quite low due to the depressed global economic conditions it seems likely that this should increase in the coming years.

If for example the price of natural was to increase to 10c for every kW/h then the payback times for both panels are reduced. This along with proper installation and maintenance of the panels throughout their useful life should hopefully mean that you recoup your money sooner rather that later.

References

1. Developed by Bob Everett & Daryl Grove, © The Open University 2003
2. http://sunbird.jrc.it/pvgis/apps/radmonth.php (accessed Oct 12th 2009)

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