Solar Thermal Collectors
Overview
A solar thermal collector absorbs heat from sunlight. The heat is then transferred via liquid to a storage tank where it can be used for a variety of household or industry needs, such as water heating, space heating and space cooling. Thermal collectors are more efficient and economical than photovoltaics for heating purposes.
Solar Collector Types
There are two main types of solar technologies used for domestic water heating: flat plate and evacuated tube collectors. Flat plate collectors are the older and more affordable type of solar thermal collectors. They are most appropriate for warm climates with a large amount of direct sunlight. The evacuated tube panel is suitable for all climates. It is slightly more efficient than flat plate panels, especially in colder climates and in the presence of clouds or strong winds. Their performance is more consistent over the course of the day and the year. However, they are also more expensive.
Placement
Solar thermal collectors are usually mounted on the roof, but can also be wall mounted or placed on the ground or on a pole away from the building. To maximize the daily performance the collector should be aligned towards the true south for the northern hemisphere, or the true north in the southern hemisphere. To maximize the all year round energy collection, the installation angle should match the latitude of the region from the horizon. That way the panel will point directly to the sun at noon during the spring and autumn equinoxes. A steeper angle of up to 15 degrees may be considered to optimize winter performance at the expense of summer surplus.
For sloped roof installations, the orientation and tilt angle will usually be determined by the roof angle. Collectors can face anywhere between south, south east and south west and have tilt angles commonly found on roofs, between 15 – 50 degrees, without losing more than 5% of the optimum annual energy collection. For flat roofs, a tilt-mounting stand can be used to set the optimal angle.
When considering the installation site for the system, make sure to pick a place that is open to sunlight during most or part of the day and throughout most parts of the year. Any shading on the collector will considerably degrade its heating performance. Any growing trees in the vicinity may also become a source of shading, and should be taken into account. To minimize heat losses, the collector should be placed close to the accumulator tank or vice versa.
Reliability
A solar collector will provide hot water as long as the sun is shining and it has a sufficiently high angle to reach the collector. In a sunny and hot region, it is feasible to size the system to provide the hot water needs for the entire year. Given a large enough accumulation tank, the system can also buffer hot water for a number of cloudy days, which makes it relatively reliable. In general though, to ensure completely reliability a solar water heating system needs to be used in conjunction with a traditional gas or electric heating system, rather than as an alternative.
Domestic hot water demand is relatively constant throughout the year. However, the solar insolation is lower during the winter, especially in colder regions. Therefore, a choice needs to be made whether to size the system to meet the full hot water demand in winter, summer or something in between. While it is possible to size a system to provide a complete all year round solution in mild and hot regions, the best return on investment occurs when the system is sized based on summer hot water usage. The system will then provide most of the hot water, most of the time. As a general rule, a system sized to meet 100% of hot water needs in the summer will provide an annual solar contribution of:
- Cold region = 50-60%
- Mild region = 60-70%
- Hot region = 70-80%
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 (pool heating), dissipating the heat using a radiator, or minimizing summer output by installing the collector at a high angle.
Durability
Thermal solar collectors are extremely durable and will essentially last as long as the building materials used to construct them. This is true for both flat plate and evacuated tube collectors. An average collector has a life expectancy of at least 30 years, while a collector built with corrosion resistant materials can last for 50 years or more. The efficiency of the collector does not degrade with time and they are built to withstand severe weather conditions. The typical manufacturer’s warranty is 10 years.
Maintenance
The collector is virtually maintenance free. Rainfall is usually adequate to keep the glass/tubes clean and soiling will typically only reduce the efficiency by up to 5%. Every 25 years or so the vacuum tubes in an evacuated tube panel will lose their vacuum, which reduces their insulating properties. The silver coloured barium layer at the top of the vacuum tube will then turn white, which gives a clear indication that the vacuum is lost. Replacing the tubes is a simple and inexpensive operation and can be done while the system is still running. Flat-plate systems do not require this type of maintenance, however should the flat plate collector be damaged repairing it will be more difficult.
The solar collector system should be checked every 1-2 years to make sure pressure is maintained and that no fluid leaks have developed. Also check that the solar controller, temperature sensors, pumps, storage tanks and other components are in working order. Circulation pumps may need to be replaced every 10-20 years, storage tanks every 15-20 years and valves every 20 years, as these are their typical lifespans. In regions requiring an antifreeze fluid, most such fluids will deteriorate over time and need to be changed every 5-10 years to maintain its freeze protection qualities. The maintenance cost of a solar water heating system is estimated at 0.5-1% of the initial cost per year.
Efficiency
The efficiency of a solar collector is impacted by optical losses, thermal losses and cosine losses.
Optical losses
Before sunlight is converted to heat the optical losses occur. Flat plate and evacuated tube collectors are insulated with glass, which is not completely transparent. The outer glass on evacuated tubes lets through 91% of the sunlight. The remaining sunlight is either reflected back (7.5%) or absorbed as lost heat (1.8%). Added to this, the absorption layer beneath the glass is 93% efficient, allowing 7% of the sunlight to be reflected back. Finally, the absorption coating has an emittance of 4.4%. Together this brings the efficiency down to 90.8% * 93.7% * 95.6% = 81%. This is the efficiency of glass-metal vacuum tubes. Glass-glass tubes has a slightly lower sunlight-to-heat efficiency at 73%, because of the double glass walls. Flat plates have only a single layer of glass, but because of its size this glass has to be of a thicker and sturdier type, and so its transmittance is only 85%. This brings down its efficiency to around 77%. There is also another flat plate version, called non-selective. This subtype uses a cheaper absorber coating and has an efficiency of 68%. A second subtype is the unglazed flat plate collector. Also called solar absorber or pool type. This collector has no glass covering it, and so its absorption factor, around 84%, becomes its efficiency.
Thermal losses
The solar collector’s thermal losses are dependent on the delta temperature, which is the difference between the collector’s absorber temperature and the ambient temperature . The warmer the collector becomes, and the colder it is outside, the less effective the collector becomes. How warm the collector needs to get depends on the application. For pool heating it will typically only go a few degrees above ambient, whereas for domestic hot water and space heating the temperature needs to reach around 60-70 ºC.
For a flat plate collector, heat is lost to the environment through convection, conduction and radiation. The higher the absorber temperature is relative to the ambient temperature, the greater the losses are and the lower is the efficiency. The difference between the two is called delta temperature. For an evacuated tube collector, the vacuum completely removes losses through conduction and convection. Only radiation losses remains, which is why an evacuated tube collector’s efficiency is less effected by the delta temperature. The graph below shows the absorber efficiency for the various solar collector types for different delta temperatures.
For solar collectors there are two ways to calculate efficiency. Efficiency of the absorption area (the area where sunlight is converted to heat energy) and efficiency of the gross area (the total area of the collector). While the absorption efficiency is interesting when comparing different solar collector technologies, the gross efficiency is more relevant for someone looking to buy a collector since it can be used to determine how much energy can be produced for a given surface area.
For a flat plate collector, the gross area and absorber area are pretty much the same, >90%. For an evacuated tube collector however, the absorber only covers 60-80% of the collector. This is mainly because there is space between the vacuum tubes and, for the double-glassed tubes, between the two glass walls. However, because the tubes are circular, this spacing will only lower the performance around midday. When the sun hits the collector at an angle, the spacing between the tubes won’t be noticeable from a performance standpoint. Some evacuated tube collectors use reflections to capture the sunlight that falls around the vacuum tubes as well. This increases the absorber to gross ratio by up to 90%, with the remainding 10% being taken up of the manifold. When considering the gross area, the collector efficiency is reduced, as seen in the chart below.
Cosine losses
The cosine losses occur because the collector is not always facing the sun. For example, when the sun is 30 degrees before or after the collector, the losses are cos(30) = 15%, relative to the performance at noon. These losses apply equally to all types of solar collectors, except for the evacuated Sydney tubes which has a passive tracking ability because of the round absorber area. This increases its performance over the day slightly.
When space is limited, a tracking system can be used to have the collector follow the sun and thereby eliminate the cosine losses. A single-axis tracker increases annual output by around 30-40% and a dual-axis tracker by an additional 6%. Performance gains vary with the season and the region. For example in California, gains ranges from 20-30% in the winter to 40-55% in the summer.
Price
The two most economical collectors for domestic hot water is the Sydney evacuated tube and the selective flat plate. Which one is more economical depends on the region where you live. Evacuated tube collectors tend to be 10-15% more expensive than flat plate collectors with the same total area.
| Type | Cost per m2 | Cost per watt |
| Evacuated tube | $400 | $0.80/w @50 ºC |
| Flat plate | $350 | $0.78/w @50 ºC |
| Plastic absorber | $25 | $0.04/w @10 ºC |
The cost per watt estimate is made at the typical operating temperature of 50 degrees for evacuated tubes and flat plates, and 10 degrees for plastic absorbers. The capital and installation cost for a residential sized system is about $4000 – $6000. This includes the collector (30%), solar water heater (25%), solar station (10%), mounts and various HVAC items. Here is a ballpark price breakdown.
| Type | Cost |
| Solar collector 4 m2 | $1500 |
| Roof mount | $200 |
| Solar station (pump/controller/expansion tank) | $500 |
| Solar water heater 500L* | $1000 |
| Pipes, insulation and connections | $200 |
| Installation | $600 |
| Total cost | $4000 |
*If the existing water heater is large enough, well isolated and has an unused heat exchanger, the extra solar water heater is not needed.
Performance
To give an example, let’s calculate how large an area of solar collectors would be needed to provide the hot water needs for a 4-person house in southern Spain.
Determining Collector Type
The average daytime temperature in southern Spain is around 20 degrees and the water heater should be kept at 60 degrees most of the time to kill bacteria. This gives an average delta temperature of 60 – 20 = 40 degrees. Looking at the efficiency chart, at this temperature difference a flat plate collector is the most economic choice. This collector will have an average efficiency of 70% at this delta temperature.
Had the temperature difference been 50 degrees or higher, an evacuated tube collector would be the better choice, despite its higher price. The evacuated tube collectors are also more efficient in regions with more cloudy weather. On a cloudy day, at 50 degrees delta temperature, an evacuated tube collector will perform at 37% of its capacity when a flat plate will only deliver 13% of its effect, relative to a clear day.
Determining Derate Factor
The formula for calculating solar energy production is:
- Energy Produced = Energy Available / m² * Area * Derate Factor
The Derate Factor is composed of a number of elements influencing the efficiency of the collector. It is calculated as follows:
- Derate Factor = Collector Efficiency * Thermal Capture Efficiency * Solar Capture Factor * Shade Factor
In this example, the following estimations are made:
| Factor | Efficiency | Comment |
| Collector Efficiency | 70% | A flat plate suffers 20% optical losses and at a delta temperature of 40 degrees the collector’s thermal losses are 10%. |
| Solar Capture Factor | 105% | Installing the collector on the roof facing south, properly inclined at 38º, it can be estimated that it will capture 1-cos(38) = 5% more sunlight over the course of a year, relative to an equally sized horizontal surface. |
| Thermal Capture Efficiency | 80% | Assuming the pipes and tank are well insulated, an estimated 80% of the heat energy will be retained. |
| Sunshine Factor | 100% | This assumes the collector is not shaded during any part of the day. |
| Derate Factor | 59% | 70% * 105% * 80% * 100% = 59% |
Determining Needed Energy
Next, the water heating energy need for the household needs to be estimated. On average, a person uses 50 liters of 50 ºC hot water per day. Assuming 4 persons live in the house, 200 liters will be consumed each day. Cold water temperature vary between 5-15 ºC, depending on the region and the season. With an average cold water temperature of 10 ºC, 200 liters of water needs to be heated 40 degrees each day to satisfy the demand. It takes 4.19 kilowatts for 1 second to heat a liter of water 1 degree. To heat 200 liters 40 degrees then takes 4.19 * 200 * 40 = 33 520 kilowatt seconds, or 33 520 / 3 600 = 9.31 kWh.
Determining Available Energy
From the insolation chart, we see that the average annual solar irradiance of southern Spain is 2000 kWh/m2/year, or 2000 / 365 = 5.5 kWh/m²/day. In the summer, we can expect more than 6 kWh/m²/day, and during the winter at least 4 kWh/m²/day.
Calculating System Size
Solving to energy formula for the area we get the following formula:
- Area = Energy Produced / (Energy Available / m² * Derate Factor)
Replacing the variables, we now have the following formula for calculating the needed size of the collector:
- Area = 9.31 kWh / (Energy Available / m² * 59%)
Dimensioning the system for the summer (80% of annual need) would require:
- Area = 9.31 / (6 * 59%) = 2.63 m²
And for the winter (100% of annual need):
- Area = 9.31 / (4 * 70% * 80%) = 3.94 m²
This means a 4 m2 flat plate solar collector would cover virtually all of the annual hot water needs for the household. The collector’s cost is around $1500 and the full system cost is between $4000-6000.
Cost per Energy
To find out the cost per energy the lifetime cost and lifetime energy production of the system needs to be determined. The initial cost of the system is $4000. Maintenance cost is very low, around 0.5% of the initial cost per year which is $20. Given a 50 year life, the maintenance costs will be $20*50 = $1000. The total lifetime cost of the system then becomes $4000+$1000 = $5000.
Assuming no heat energy was wasted, the system would annual produced this much energy: 2000 kWh/m2 * 4 m2 * 59% = 4700 kWh. During its 50 year life expectancy, the system will produce 4.7MWh * 50 = 200 000 kWh. The cost of energy then becomes $5000 / 200 000 kWh = $0.025/kWh. This is almost 5 times cheaper than US electricity prices at $0.12/kWh.
Advantages
- Low cost – Solar collectors is the cheapest form of renewable energy.
- High efficiency – Solar collectors have among the highest efficiencies of all energy generating technologies.
- High durability – The life expectancy of solar collectors are extremely high.
- Low maintenance – Solar systems will run for many years without maintenance.
Disadvantages
- Low reliability – Solar heat energy is not produced at night and is greatly reduced in cloudy conditions.
- Large surface area – The more energy you need, the larger surface area is required.
