There has been quite a bit of hubbub in recent years regarding solar technology as the principal form of reducing our dependency on traditional fossil fuels. With the exponential growth of this industry throughout every corner of the world, people have tended to focus on photovoltaic (PV) technology without thinking as much of its cousin, solar thermal.

Unlike PV, its cousin is used specifically to convert solar radiation into usable heat energy instead of electric power. Its main applications include hot water and heating both for residential as well as commercial use. In addition, there is also a significant amount of innovation currently to harness solar heated water for solar powered air conditioning.

EFFICIENT AND ECONOMICAL

What is most impressive about this technology is the level of efficiency that can be reached without hardly impacting your wallet. By only taking up one tenth of the space of a PV system for a quarter of the price in the majority of cases, a solar thermal system can reach efficiency levels of up to 75% whereas PV systems only reach around 20%. Therefore, the payback is much greater than PV.

Up until today the preferred choice of solar thermal collector in Europe has been the flat plate, however, I would like to briefly focus on the advantages of the evacuated tube with a round absorber, which has become the preferred type of solar thermal collector in China which comprises 70% of the market. Why has this collector become the collector of choice for the largest solar thermal market worldwide?

THE EVACUATED TUBE: ROUND ABSORBER AND IAM

Due to the fact the evacuated tube has a rounded absorber layer, the absorber surface is always exposed to direct solar radiation from dawn until dusk unlike flat plates or evacuated tubes with flat absorbers, which only allow direct solar exposure during a couple of hours in the afternoon. This is known as the incidence angle modifier (IAM).

The following graph demonstrates that the y-intercept is really not the key indicator of performance for a collector due to the fact the IAM factor allows for a more stable solar output throughout the entire day. Focus on the blue curve:

Flat Plate	Flat Absorber	Round Absorber

EASE OF INSTALLATION

An evacuated tube collector comes in various parts (manifold, frame, tubes) all of which are assembled on the roof making it much easier for the installer and reducing the overall installation since many flat plate installations require a crane. In addition, the tubes are extremely resilient and can be easily replaced without affecting the integrity of the system flow. This means the system keeps functioning despite the fact a tube may have broken...something which cannot be achieved even with the best flat plates.

VACCUUM AS AN INSULATOR

Due to the insulating properties of the vaccuum there is little heat loss. This is necessary for reaching the temperatures required for various industrial and commercial applications. The vaccuum functions like a thermos, thus allowing the evacuated tube collector to yield temperatures of up to 90C even in cold or cloudy weather. Flat plates cannot achieve these temperatures due to the fact that there is no vaccuum to prevent a massive amount of convective heat loss as the delta-T is increased.

THE FUTURE

The interesting thing is that despite the fact evacuated tubes have better performance throughout the day because of IAM, are more economical due to ease of installation, and are capable of reaching higher temperatures, the technology has not received the recognition it deserves since the majority of the publicity we currently see focuses on PV. Why? I am not exactly sure. Perhaps PV is more trendy or sexy. Nonetheless, its cousin, solar thermal, must not be left to sit in the shade.