Integral collector storage systems

For integral collector storage systems (ICS) the hot water storage tank is integrated with the collector itself. A technically robust system is not easy to

Table 3.4 Heat Transition Coefficient k and Total Energy Transition Coefficient (g-value) of Various Conventional Materials and Transparent Insulation Materials (TIMs)

Material

Conventional glass covers k in g-value W/(m2 K)

TIM between low-iron glass with air gap

Material k in g-value

Material

Conventional glass covers k in g-value W/(m2 K)

TIM between low-iron glass with air gap

Material k in g-value

1 layer of glass (4 mm)

5.9

0.86

Aero gel granulate (20 mm)

0.85

0.4

2 layers of insulating glass (20 mm)

3.0

0.77

Polycarbonate honeycomb structure (100 mm)

0.7

0.66

3 layers of glass with IR coating (36 mm)

1.0-1.2

0.53-0.62

Polycarbonate capillary structure (100 mm)

0.7

0.64

Note: IR = infrared reflecting

Note: IR = infrared reflecting construct. If the system is used in regions with a danger of frost, heat losses cool both the collector and storage; ultimately, they can freeze and be damaged. A double-circuit system with frost protection is not possible to realize as an integral collector storage system. A way needs to be found for achieving significant reduction of the collector heat losses. Better insulation on the back is not a problem - the problem is heat losses through the front cover. Sunlight must pass through the front with low absorption and reflection losses. The cover must therefore be transparent, and yet this leads to large heat losses through the cover. A vacuum can reduce the heat losses, but not as much as is necessary to design an integral collector storage system.

New so-called transparent insulation materials (TIM) brought a solution to these problems (Lien et al, 1997; Manz et al, 1997). These materials have a slightly lower transmittance compared to low-iron solar safety glass. However, the heat transition coefficient is significantly lower so that the heat losses are reduced to levels acceptable for ICS systems. Table 3.4 compares various conventional and TIM covers.

Figure 3.5 shows a sketch of an ICS system. The hot water tank is made of stainless steel. The back is perfectly insulated. Reflectors on the inner side of the back reflect the light to the storage, which is also the absorber of this system. The transparent insulation material is under the glass front cover. A system covering two square metres has a storage volume of about 160 litres.

ICS systems do not need an external heat storage tank, which is necessary for other collector systems. The total system is simpler: some components found in other collector systems are not necessary and this reduces the cost. If the water temperature in the storage tank is too low, an auxiliary, thermostat-controlled water heater can boost the temperature up to the desired level.

Integrated Collector Storage Ics
Figure 3.5 Cross-section through an Integral Collector Storage System

Disadvantages of the ICS systems are the high weight and large dimensions. This makes installation more difficult in many cases. Furthermore, the system efficiency is usually lower than that of an optimal system with forced circulation. These are some of the reasons why the integral system type has not reached a high market penetration to date.

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