A WISC — Wind and Infrared Sensitive Collector — is an unglazed solar collector whose output depends measurably on wind speed and long-wave sky radiation, and which is therefore tested under ISO 9806 with wind- and infrared-dependent coefficients. Lacking a cover, it exchanges heat directly with the air and can even harvest ambient and condensation heat below air temperature, making it the standard collector category for heat pump source duty.
What makes a collector "WISC"?
A glazed collector hides its absorber behind glass, so wind barely touches its performance. Remove the glazing and everything changes: the bare absorber trades heat directly with moving air and with the sky, so wind speed and long-wave (infrared) radiation exchange become first-order effects rather than rounding errors. ISO 9806 groups such collectors under the WISC label — Wind and Infrared Sensitive Collector — covering classic unglazed pool absorbers, uncovered selective-surface collectors and uncovered PVT hybrid collectors. The defining feature is not a missing pane of glass in itself but the physics it unlocks: a WISC is as much an air-coupled heat exchanger as a solar device.
How is a WISC collector tested under ISO 9806?
ISO 9806 tests all collector types with one unified method, but for WISC devices the performance model adds terms that glazed collectors omit: coefficients describing how efficiency varies with wind speed and how the collector exchanges long-wave radiation with the sky. Test conditions include defined wind speeds over the aperture (test points around 1.3 and 3 m/s are used in the standard's procedure), and the resulting datasheet states output as a function of irradiance, ambient-to-fluid temperature difference and wind. On a Solar Keymark certificate this is why an unglazed product's table looks different from a flat plate's — the meaning of each entry is unpacked in What do "WISC" and "unglazed" mean on a Solar Keymark certificate?
Why are WISC collectors good heat pump sources?
Because they work below ambient temperature. Run the fluid a few kelvin colder than the air, and a WISC absorbs heat from three sources at once: solar radiation when available, convection from the surrounding air day and night, and the latent heat of dew or frost condensing on the surface. A glazed collector cannot do this — its insulation blocks exactly the air-coupled gains a heat pump source wants. Feeding a brine/water heat pump from an uncovered collector field turns the array into a silent, defrost-light outdoor unit substitute; the same principle underlies PVT panels as a heat pump source, where the cells provide electricity while the thermal side feeds the evaporator. Uncovered PVT models are certified precisely in this WISC class; Solimpeks has produced PV-T collectors since 2008.
WISC vs glazed collector: which for which job?
| Property | WISC / unglazed | Glazed flat plate |
|---|---|---|
| Peak fluid temperature | Low — near ambient | High — 60–100 °C+ |
| Works below ambient (heat gain from air) | Yes | No |
| Wind sensitivity | High — tested with wind terms | Low |
| Stagnation risk | Minimal (low stagnation temperature) | Real — 160–200 °C typical |
| Typical duties | Pool heating, heat pump source, preheat | DHW, space heating support |
The two categories are complements, not competitors: a glazed collector is a temperature machine, a WISC is an energy-harvesting surface. For domestic hot water at 60 °C, choose glazing; for feeding an evaporator at 0–20 °C or heating a pool at 25 °C, the uncovered collector wins on cost, weight and yield per invested euro.
Does a WISC collector work at night and in winter?
At night, yes — as long as it runs below air temperature, it keeps absorbing convective and condensation heat, which is exactly how PVT- and unglazed-source heat pump systems operate through the dark hours. In winter the balance tightens: air is colder, so source temperatures fall and system efficiency with them, and frost forming on the absorber must be tolerated or melted. Sizing the collector area to the heat pump's evaporator demand — rather than to summer sunshine — is the core design task, treated in Can PVT panels be the only heat source for a heat pump?
Frequently asked questions
What does WISC stand for?
Wind and Infrared Sensitive Collector. It is the ISO 9806 category for unglazed collectors whose performance depends significantly on wind speed and long-wave radiation exchange with the sky, so their test model includes wind- and infrared-dependent coefficients.
Is a WISC collector the same as a pool absorber?
Pool absorbers are the oldest WISC family, but the category is wider: it includes uncovered metal collectors with selective coatings and uncovered PVT hybrid panels. What unites them is the missing cover and the resulting air-coupled behaviour.
Can a WISC collector produce domestic hot water?
Not efficiently on its own — without glazing, losses climb steeply once the fluid runs much hotter than the air. WISC collectors shine as preheaters, pool heaters and heat pump sources; a heat pump then lifts their low-grade heat to DHW temperature.
Do WISC collectors get Solar Keymark certification?
Yes. Solar Keymark certifies unglazed and WISC collectors, including uncovered PVT, based on ISO 9806 testing. The certificate annex reports their performance with wind-speed-dependent coefficients rather than a single glazed-style efficiency curve.