The heat transfer medium water

Since 2003 PARADIGMA has been using water as heat exchange medium with the so called AquaSystem. Frost protection is ensured by unutilized low-temperature heat from the installation. Because of the low heat losses of the CPC vacuum tube collectors only a small amount of energy (about 2 to 4 % of the yearly gain in energy) is required for frost protection and this is more than compensated by the advantages of using water and high operating temperatures. This concept has proven successful more than 30,000 times so far.

The advantages of the AquaSystem over conventional solar collector systems are obvious. The operation with water:

allows easy, direct connection to the in-house heating network,

requires less equipment such as heat exchangers, de-aerators, valves, pumps and controllers,

is the precondition for the use of efficient and small storage tanks,

eliminates the high costs for antifreeze and the associated running costs,

reduces considerably the costs and time for commissioning and repairs,

ensures a long operating life with almost constant performance,

removes all risks associated with thermal stagnation and

minimizes running costs (e. g. maintenance).

The use of state of the art CPC vacuum tube collectors ensures the maximum harvesting of solar energy with a system that has a long operating life and is virtually maintenance free.

It is absolutely unscientific that the power investigations according to DIN EN 12975 (see chapter 7 “power performance…”) were carried out with water without any consideration of the heat transfer fluid propylene glycol which is normally used. Glycol has with 40 °C compared with water only 88 % of the heat capacity, 3.8 times the viscosity, 62 % of the heat conductivity, only a quarter the Reynolds number (therefore these collectors must work at unfavourable, laminar flow conditions), 75 % of the heat-transfer coefficient and up to 3.85 times higher pressure losses. And the deeper the temperatures are the worse the glycol conditions. A water-glycol heat exchange requires up to 3 times the heat exchange area than a water-water heat exchange to get the same NTU (number of thermal units). Nearly all simulations programs for solar harvest and almost the whole solar literature disregard this physical context, e. g. by use of too simple models.

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