TrendSetter Premium Solar Thermal Collector & System Sizing
The wide range of sizing options of the TRENDSETTER® Premium Solar Collector allow for heat outputs to be better matched to requirements. The sizing of a system is usually associated with the size of solar storage tank.
Typical daily sizing requirements for apartments is 20 gallons for the first bedroom, 15 gallons for the second bedroom and 10 gallons for the third bedroom.
Typical minimum daily sizing for residential homes is 26 gallons per adult and 13 gallons per child. Add additional usage for a washing machine and dishwasher.
Therefore, it is common to estimate the average family consumes on the average of 80 gallons of hot water per day.
Sizing Example
- Household using 80 gallons of water per day @ 114ºF
- Summer cold water temperature of 65ºF
- Average summer insolation level of 1,902Btu/ft²/day
- Determine temperature rise
114 – 65 = 49ºF temp rise
- Determine energy requirement
80 gallons x 8.33 lbs/gal = 667.2lb
667.2lb x 49ºF = 32,692Btu
- Determine solar collector output / tube
1902Btu * 70% conversion =
1331Btu per ft² of collector absorber area
1331 * 0.86ft² absorber area = 1144Btu/tube/day
- Determine tube requirements
32,692 ÷ 1144 = 28.6 tubes
From this sample calculation, it is shown that a 30 tube collector would suit this household perfectly, providing 100% of their hot water needs in the summer and a percentage throughout the year depending on hot water usage patterns and solar radiation levels.
As a general rule, meeting 100% of hot water needs at peak production in the summer will provide an annual solar contribution of:
- Cold region = 50-60%
- Mild region = 60-70%
- Hot region = 70-80%
Energy Output Calculations
In most cases a rough estimation is all that is needed and therefore simple calculations can be used, but it is also of great benefit to understand performance calculations. The followings sections aim to provide a basic overview of calculating collector performance.
Calculating Instantaneous Output
To calculate the output of the solar collector the following factors are required:
- G = Insolation level (W/m²)
- Temperature differential
- Tm = water temperature (being fed to collector)
- Ta = Ambient temperature
- Collector Performance Variables (based on absorber area)
- no (y-intercept; Trendsetter Solar = 0.717)
- a1 (first coefficient of loss; Trendsetter Solar = 1.52 W/m²W)
- . a2 (second coefficient of loss; Suntech Solar = 0.0085 W/m²W²)
- X = (Um-Ta)/G (this is sometimes written as T*m)
The following formula is then used (metric calculations):
Performance = no – a1 * X – a2 * G * (X)²
Example:
- Solar insolation level of 800Watts/m2
- Supply temp to collector from bottom of storage tank of 35ºC
- Ambient temperature of 25ºC
- Direct angle of incidence (i.e. Midday)
Therefore: G = 800; X = 0.0125
Plugging those figures into the formula:
- 0.717 . 1.52* 0.0125 . 0.0085 * 800 * (0.125)2
- 0.717 . 0.019 . 0.0011 = 0.6969 = 69.7% conversion efficiency
Therefore given 800 Watts of solar radiation, the output of the solar collector would be 557 Watts – this is per m² of absorber area. If the calculation was made using collector performance variables based on aperture or gross area, then the output would be based on that sizing.
The absorber area of each TRENDSETTER® Solar evacuated tube is 0.08m² so a 30 tube collector is 2.4m².
The value of 557 Watts should therefore be multiplied by 2.4 to obtain the output value for a 30 tube collector.
557 x 2.4 = 1336 Watts or 1.336kW.
Considering Incidence Angle Modifier (Iam)
The above calculation has considered instantaneous heat output, and at midday when IAM is 1. Given the advantages of the round shaped tubes, and resultant favorable IAM curve, heat output can actually exceed these levels either side of midday.
Calculating Daily Or Annual Output
If the calculation of solar collector output over a period of time is required, such as over a day, month, or year a very simple calculation can be made. These calculations are however very general, and are recommended if a fast estimate of heat output is required, such as when sizing a system. For more accurate estimates please use a modelling program such as F-chart or TRYNSYS.
The following calculations are converted from metric values by using the following conversions:
- 1 kWh/m² = 316.74 btu/ft2 and
- 1 kWh = 3409.4 btu
Daily Calculation
When considering all factors, the following average solar conversion values can be used for TRENDSETTER® Solar collectors:
- Cold Blue Sky Winter Day = 60%
- Mild Blue Sky Spring/Fall Day = 70%
- Hot Blue Sky Summer Day = 90%
- On cloudy overcast days reduce above by 30%
Using the average daily summertime solar isolation level, the following calculation can be completed.
For a south facing solar collector situated at 40ºN Latitude on a 30º tilt on June 21st the average daily solar insolation value is 7.68 kWh/m²/day. Multiplying by the conversion efficiency of x 69.7% and a 90% summer day factor yields a collector output of 4.8 kWh/m²/day or (1520 btu/ft2 /day)
The net daily output for a 30 tube array is 4.8 kWh/m² x 2.4 m2=11.5 kWh or 25.8ft2 x 1520 btu/ft2 = 39,216 Btu
Annual Calculation
When considering all factors, the following average solar conversion values can be used for TRENDSETTER® Solar collectors:
- Cold Region = 60%
- Mild Region = 65%
- Hot Region = 90%
The cumulative twelve month solar production for a south facing 30 tube solar collector situated at 40ºN Latitude on a 30º tilt solar insolation value is calculated to be 2430kWh/m²/year x 0.697 x .65 x 2.4m2 = 2642kWh. Multiplying 2642kWh by the conversion factor of 3409.4 btu/kWh = 9.0Mbtu
Gas is billed in therms equivalent to100,000 btu. In the above example, assuming an energy factor of 50% the annual savings is 180 therms.
Do Not Oversize The System
Ideally size the system based on summer solar radiation levels and hot water usage. Sizing for winter will result in excess summer heat, which must be safely used or dissipated. There are various system designs that can allow for an oversized system including using the heat for auxiliary heating (spa/hot-tub/pool), dissipating the heat using a radiator and/or tank cooling function, or minimizing summer output by installing the collector at a high angle.
Pipe Insulation
Copper pipe (as commonly used on the solar loop) is a fantastic natural heat dissipater, and therefore if not well insulated the system performance will be greatly reduced, due to passive heat loss. We recommend the use of our Rork Stainless Steel flexible pipe sizes supplied in 10 meter rolls to cut to length as required. A tool
allows the flange to be made where required. ALL exposed piping both indoors and outdoors should be heavily insulated, regardless of the climate.
Installation Location
Installation angle (inclination) and direction (azimuth) are both important factors, which should be considered. As a general rule the installation angle should match the latitude of the region, however this may not be the case if the system design needs to minimize summer output.
The azimuth should be as close to due south as possible. Pointing towards the East or West by up to 30° is acceptable and will have minimal impact on the collector output. Pointing the collector at greater angles towards East or West will change the peak period of heat output to morning or afternoon respectively.
Make certain the collector will not be shaded significantly during the day. Early morning before 10AM or later afternoon shading after 3 PM is not a major problem as solar radiation levels are low during these periods anyway.
Safety
Adhere to all relevant safety regulations when completing the installation, in particular health and safety guidelines regarding working on a roof/ladder. Ensure the collector will not become a hazard in the case of damage. E.g. during a storm if flying debris strikes the collector, tubes may be broken, resulting in broken
glass. A consideration of where such glass may fall should be made – if on an area where people re likely to walk, safety measures may need to be employed. E.g. installs a guard below the collector, and/or educates the home-owner.
TrendSetter Disclaimer
TRENDSETTER® maintains the right to change dimensions and the characteristics of the product without notice, and is not responsible for misprints. This booklet is only a guide and as such TRENDSETTER® will not be held responsible for the performance of the system in which its collectors are used or responsible for any damage to person or property that results during the installation or subsequent use of TRENDSETTER® products and related system components. In all cases a thorough understanding of local regulations, laws and common practices must be made, and adherence to such ensured, before commencing the design or installation of any system incorporating TRENDSETTER® products.
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