Recusol™, central air heater

A larger comfort is performed if the actual excess of sun heat is stored in a heat storage by an extra heat exchanger before the air enters the ventilation system of the building. This stored heat can be used at a later time if there is heat shortage. In the feasibility study it was found that monthly storage is sufficient. For this short period the excess heat can be stored directly under the building by small U-tubes pulsed around the building. Due to the short period the same storage system can be used in the summer for the storage of cold from the night air, which can cool the building during hot periods.

To avoid dirt the inlet and outlet of the Recusol™ should be filtered. Possible still present micro-organisms will be killed by regularly available high sun intensity, while with the transparent Recusol™ dirt is easy visible during inspection.

The transport channels in the Recusol™ are designed to remove condense; while possible ice depletion will be removed by temporarily turning around the flows, such that the ice will melt by the warm waste air.

Proof of principle

Before starting further development a simple proof of principle was executed by Innovy in Alkmaar. A test sample is produced from adapted polycarbonate multi wall sheets in which small air distribution holes are stabbed with a pin. After that new channels are assembled by gluing the adapted channels with thin polycarbonate plates used for the heat exchanger. The small sample is placed in a test rig representing a down scaled building with a scale of 1:67.
Because standard available multi wall sheet is applied with 8 mm wide channels it was not possible to fabricate the optimal design with 5 mm wide channels. If 5 mm wide, there are 200 channels per m with a K-value of 600 W/K per m² Recusol™, which is very large for an air to air heat exchanger. If 8 mm wide, the K value is 234 W/K per m² Recusol™, which is still rather high for an air to air heat exchanger but 60% less than for a 5 mm channel.

Because the heat exchange channels were glued instead of extruded, one channel leaked and effectively 2 of the 19 channels were blocked by rest glue material, so 86% of the sample did operate well. Also due to the glue the sample was less transparent: around 40% of the light went through the sample and was absorbed by the black absorber, around 20% was reflected and around 40% was absorbed by the heat exchange channels and transport channels.

The test rig represents a well insulated (12 cm thick glass wool and HR++ windows) standard family house (a house between two other houses) with the characteristics given in the table below:

The internal heat sources are electric equipment, people, cooking, passive (sun) light through windows, etc.

A test performed around the shortest day of the year is shown in figure 7.

From the test can be concluded that the recovery efficiency of the waste air heat is between 80 and 95%, which is rather high for a far from perfect test sample. The solar absorption efficiency is 50%, from which 40%-point is recovered by the heat absorber and 10%-point by the heat exchanger, absorbed by the not perfect transparent channels, while approximately 22%-point is reflected by the not perfect transparent channels. The remaining 28%-point is lost in the waste air exhaust of the Recusol™.
It is thought that if the transport channels were designed in counter flow more absorbed heat of the remaining 28%-point will be recovered. In that case the solar absorption efficiency will be 60%.
If the test sample is extruded instead of clued the channels would be more transparent and also less light will be reflected. In that case with the application of counter flow in the transport channels it is expected that the solar efficiency will be 70%.
Never the less the test with the glued sample shows that the Recusol™ is moderate sensitive on dirt, because part of the (sun) light heat absorbed by the dirt will be recovered by the heat exchanger. Also because the absorber is very well insulated (comparable with 24 cm thick glass wool) the efficiency is hardly sensitive to the outside temperature.

In figure 8 a test is shown over a longer period, which was rather cold for Alkmaar.

The “house in” temperature was maximal 65 ºC and would be higher if the leaveless tree from the neighbours positioned in the south was not present. If the heat was stored the temperature in the test rig (house) would be between 22 ºC and 24 ºC and sufficient for this cold period.
This confirms that a short storage period is sufficient as well.

Cost estimation compared with conventional central heating

Based on series production the estimated costs for a standard family house are:

For a standard family house 49% on costs are saved.
Also 2500 kg CO2 is saved per year.
If the house is poor insulated (K=value 184 W/K instead of 92 W/K) the total costs are resp. for the Recusol™ 1006 €/y and for the conventional central heating 1603 €/y, which is 38% cheaper and significant too. However compared with the well insulated house 500 kg more CO2 per year is used, so for the environment it is recommended to apply a well insulated house.

Temperature scenario´s

Based on measured temperatures and sun input at De Bilt the Netherlands the temperature in a standard family house is calculated for the years 2001 up to the beginning of 2007.
Figure 9 shows the temperatures and possible extra heating by an extra mood heater during the heating season. Figure 10 shows the temperatures during the cooling season.

The heating by the 20 m² Recusol™ and the storage (30 m³ soil) is sufficient to heat the house, only in 2003 and 2006 a small 1000 W mood heater would have been needed with a mean consumption of 47 m³/y NG. If a 10 m² Recusol™ is applied in all years an extra heater of 2000 W would have been needed, with a mean consumption of 140 m³/y of NG.

With the storage of cold from the night air, only for a short period in 2003 and 2006 the temperature of the house exceeded 25 ºC.

Critical points

Critical points are:
• Dirt scale on the Recusol™ surfaces
• Condense
• Ice

Most of the dirt will be removed by the filters placed before the waste and the fresh air input. Because dirt also absorbs heat, from which a large part is recovered, the Recusol™ is not that sensitive to dirt. Possible micro organisms will regularly be killed by the heat of the sun, while due to the transparent Recusol™ dirt can easily be detected and possibly break down by UV light or special gasses.

In the heat exchange channels special drains will be applied which removes the condense to the bottom, where it should be tapped regularly.

If ice is present it can be removed by turning around the flows inside the Recusol™, such that the cold side will be heated by the hot input waste air, which will melt the ice.

© 2009 Innovy