July 05, 2018

Solar Hydronic Collectors ratings

Measuring Solar Collector's thermal efficiency is a complicated and expensive task. This why the Solar Rating and Certification Corporation (SRCC) and the Canadian Standard Association (CSA-F378) have accredited various labs in the US and Canada to provide more accurate, consistent and standardised efficiency values.

What defines collector's thermal efficiency and thermal energy outputs are the following factors:

Ti : Collector's entering fluid temperature in ⁰F or ⁰C.

Ta : Ambient Dry Bulb temperature in ⁰F or ⁰C.

I or G: Insolation Rate expressed in Watts/m² or Btu/hr.ft².

Type of Collector: Flat Plate (Glazed and unglazed), Evacuated Tubes, etc...

Collectors' thermal efficiency (Which is no more than the ratio of thermal energy output divided by solar insolation) is then drawn as function of (Ti - Ta)/I (or G)

 Why Evacuated Tubes Heat Pipe Solar Collectors Are Among The Most Efficient In Nordic ClimatesFig.1

As we can see in Fig.1:

  • Solar Collector's thermal performance decreases when Ti (entering fluid temperature) is higher and/or Ta (ambient temperature) is lower.
  • Solar Collector's thermal performance increases when Ti (entering fluid temperature) is lower and/or Ta (ambient temperature) is higher.

So in hotter climates, where (Ti-Ta) is low, the best performing collectors are the one with a steeper inclined performance curves and in colder climates (such as in Canada and North of the United States) the best performing collectors are the one with a flatter performance curve.

How do various Hydronic Solar Collectors perform?

 Why Evacuated Tubes Heat Pipe Solar Collectors Are Among The Most Efficient In Nordic ClimatesFig.2

  • The blue Line reflects unglazed solar panel performance. They offer the steepest (Highest Y intercept value) curve, meaning they perform very well the entering fluid temperature is low and ambient air temperature is high (warm air). The steep slope shows that decreasing air temperature (or increasing entering fluid temperature) take a big toll on performance. This type of solar collector is great for heating pools in the summer because pool temperatures are usually fairly close to daytime air temperatures in the summer when pool use occurs (Ti-Ta is at its minimum, and efficiency is at its max).
  • The red line reflects glazed flat plate solar panel performance. They offer slightly lower Y intercept values because the glass coating reflects some light, but the efficiency slope is flatter than unglazed panels because the glazed panels have insulation inside and trap the absorber's heat inside the collector's box. as a result the panel looses less heat when outdoor air is cooler.
  • The green line represents evacuated tube performance. These collectors offer lower Y intercept values and their slope is even flatter that glazed panels. On sunny hot days , they don't perform as well as flat plates, but the excellent vacuum insulation makes them more efficient that flt plates when it's very cold outside or sun is weak (low insolation values). This flatter slope also means that evacuated tubes are more appropriate for projects requiring high fluid temperatures in colder climates such as space and process water heating.

What is the Useful collector's thermal efficiency?

Having free hot water from your solar panels is fun but to payback your investment (the cost of panels, storage tank, piping, control, etc...) you need to make use of the generated heat. Living in Canada and having a lot of hot water in July and August in excess of your domestic hot water demand is simply a waste. The reason is simple, a solar generated unused heat, is a wasted heat that don't count in your installation annual efficiency, because that wasted heat do not offset any KWh from your utility bill.

The intersection between the Green and Red performance line in Fig.2 is at (Ti-Ta)/G=0.4. Assuming we are in Montreal where:

  • Outdoor temperature is at -30⁰ C and our Solar Panels are used for space heating where the heating loop is heated from 104 ⁰F (Ti=104 ⁰F=40⁰C) to 120 ⁰F. In half sunny half cloudy day with G=500 W/m². = 158 Btu/hr.ft² (Ti-Ta)/G= (104+20)/158=0.78 way on the right of the intersection point. This gives us a thermal efficiency of 0.35 or 35% for vacuum tube collectors versus a 0.1 or 10% efficiency for glazed flat plate collector.
  • Outdoor temperature is at 0⁰C and our Solar Panels are used for space heating where the heating loop is heated from 104 ⁰F (Ti=104 ⁰F=40⁰C) to 120 ⁰F. In half sunny half cloudy day with G=500 W/m². = 158 Btu/hr.ft² (Ti-Ta)/G=  (104-32)/158=0.45 way on the right of the intersection point. This gives us a thermal efficiency of 0.4 or 40% for both types of collectors.
  • For outdoor temperatures below 0⁰ C, Flat Plate solar collectors are more efficient but heating demand is much lower than in extreme winter conditions. Higher Thermal efficiency at mid-winter season generate more energy than needed, thus increasing the volume of wasted energy

In the city of Montreal we have 121 110 Heating Hours for outdoor temperatures below 0⁰ C (where heating demand is high to medium) and 54 385 Heating Hours for outdoor temperatures above 0⁰ C (where heating demand is moderate to low).

Running an Excel Bin Hours Energy simulation and comparing our XKPH58 Vacuum Tubes Solar Collectors (3 x 30 Tubes Collectors) to a Glazed Flat Plate solar collectors (having a Y intercept of 0.792, slope of -7.517 W/m².⁰C and same Gross Area) and after putting the monthly Generated KWh and the Building Monthly Heating Demand into a graph, we get the following result:

 Why Evacuated Tubes Heat Pipe Solar Collectors Are Among The Most Efficient In Nordic Climates Fig.3

As we can see in Fig.3:

  • Between the months of April and end of October, Glazed Flat Plate Collectors generate much more energy than Vacuum tubes Collectors in a scale exceeding Building Hot Water demand. Generated Hot Water in Excess of demand is not counted toward annual Collectors' thermal efficiency.
  • Between the months of November and end of March, Glazed Flat Plate Collectors generate much less energy than Vacuum tubes Collectors in a scale far below Building Hot Water demand.

Adding all together, the Useful Generated Hot Water energy by the vacuum tubes collectors is 13 800 KWh Vs a Glazed Flat Plate generated energy of 11 474 KWh. This means that in this case Vacuum Tubes Collectors are 20.3% more efficient than Glazed Flat Plate.


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