Posted by Mike Holm on March 27, 2010
PVThermal panels are not new. There has been a lot of research done on them in Europe and the US since the 1970s, but they have been a niche market. Now is the time for the system to gain some prominence.
The basics of the panel are quite simple. A heat exchanger, similar to that in a regular flat solar water heating panel, made of copper tubing and aluminum sheeting is placed behind the PV cells.
Water or glycol is piped to the panels in the same way as any hot water solar system. The solar panels in this type of system can only get to around 70-80C (158-176F). Because of this limiting factor the glycol will never boil and will therefore last a very long time, maybe 15-20 years. In a drainback system the operation is the same as in a traditional system but the installer must remember to tilt the panels to allow for the liquid to drain back. Most of the reasons for using a drainback system over a glycol system disappear because the drainback’s main advantage is to prevent liquid boiling and degrading quickly.
The main reason why the temperature cannot get as high as a standard panel is that there isn’t the insulating effect of a glass cover. In this way it is similar to a pool panel. The limiting effect of the heat loss over the PV glass keeps the temperature down.
From an electrical point of view, a PV panel can have a 20-25% reduction in efficiency when the cell temperature is at 80C. This amount of heating happens quite regularly in the hot summer. If we can reduce the temperature of the panel to 25-30C, we can expect to see an annual improvement or 10-15% in electrical output. With a standard 2kw PV system, for example, the payment under the Ontario microFIT program will be approximately $2000. Therefore with the PVThermal system we should see an extra $200-300 annually.
The hot water production is equally impressive. The average solar water heater is 5-6m2. With a 2kw PVThermal system there is 15m2 of panel area. The panel is not as efficient (per m2) as a standard panel, but there should be no shortage of hot water during the prime solar months of March to October. During the colder months, performance per m2 will be less than a standard panel but the extra surface area allows for some heat for other purposes.
One other benefit of the the PVThermal comes when snow or frost has covered the panels. Some solar controls will allow the panels to be defrosted by warm water in the storage tank which could give you electrical production when otherwise you would have to rely on the grid. More coming soon including diagrams and sizing info.
Posted by Mike Holm on June 12, 2009
Vacuum tube collectors
There are a lot of vacuum tube collectors (i.e. solar panels) available on the market these days, and there is definitely a lot of misinformation out there. I shall try to rectify this to some extent.
“Vacuum collectors are more efficient than flat collectors.” TRUE or FALSE?
False most of the time and true some of the time. A good flat collector will start off with a higher efficiency than almost any tube collector (with a few exceptions from Europe). During the summer, or whenever the collector is operating at less then 60C above the outside temperature, the flat collector will be superior to the vacuum collector (different collectors have different characteristics and efficiency curves so 60C is a bit of a generalization)
The time spent below a delta T (temperature difference) of 60C is actually the majority of the time for domestic water heating systems. It is only when it is -10C outdoors for example that the vacuum collector will outshine a flat collector, and due to the range of collectors out there this is not true of all collectors.
Flat panel collectors on a private home
Here is an example: a Viessmann flat collector, at its most efficient (no heat loss through the enclosure of the collector) is about 79% efficient. When the outside temperature is 60 deg colder than the collector temperature, the efficiency drops to about 58%. This range is where the collector will spend 75% of the time. By contrast a good vacuum collector coming from China using the Sydney type double tube (the most used type of tube) has an initial efficiency of 60%. At 60C delta T the efficiency is 50%. The efficiencies of the collectors will meet when the temperature difference is closer to 80C. It is only in the dead of winter when this will happen and we don’t have a lot of energy in the sun at that time of year.
Another real issue is longevity. The Sydney tube is only about 20 years old and has only seen the light of day in North America and Europe for 6-8 years. As a matter of fact, of the hundreds of tube suppliers in China, only a small fraction will meet any certification. Most operate with no manufacturing standards controls. The market in China is much different than here. The tubes were designed to last 7-8 years and could be replaced one at a time if one broke. The economics of life in China dictated that the tubes needed to be cheap enough to grow the market. Few will last 20-25 years and not without a tube or two being replaced annually. The vacuum tube works under a lot more structural stress than a flat collector so it is natural to expect that it will not last as long.
Many flat collectors, by contrast, have been running for 25-30 years and show no sign of dying. More to come on this topic.
Here is an article from Germany printed a few years ago. There have been moderate improvements in both flat and tube collectors since then but nothing that would change the essence of the paper. Performance Test – Flat Plate and Vacuum Tube Collector
