Solar Drying
Function principle
FAQ Usage of thermal energy

FAQ Usage of thermal energy

Usage of external thermic energy (waste heat) in the drying halls for solar sewage sludge drying

Basically, the following aspects have to be considered in this complex matter:

  1. Dewatered sewage sludge consists mainly of water (70-80%), which is poor thermal conductor.
    Result: Poor heat transfer requires high temperature gradients for an efficient utilization.
  2. During the solar drying process a granule is formed starting at approx. 60% dry matter content. This granule has a density and a thermal conductivity similar to wood. Between the grains of the granule is air. If you take into account the thermal conductivities of the air and the granule as well as the granule`s bulk density it turns out that the material forms an isolation similar to polystyrene.
    Sewage sludge granule acts as an isolator.
  3. Solar dryinghalls are constructed with a building envelope as transparent as possible. In doing so, materials such as polycarbonate sheets, foils or glass are used. The U-value of the utilized materials is usually 3-3.6Wm-2K-1. The better the U-value the worse the transparency and therefore the less solar evaporation.
    About 50% of the used thermal energy is lost through the building envelope.
  4. The air exchange rates in solar sewage sludge drying are as high as 60-100m3 air per m2 drying area and hour. This air and the water it contains have to be heated and then get transported out of the drying hall.
    approx. 20-25% of the used thermal energy get lost due to the air exchange.

  5. During the solar drying with the SolarTiger® technology the granule can reach temperatures up to 55°C and more at the surface. Therefore, the usage of floor heating systems requires a high temperature gradient (cf. a.) and a low sludge height (cf. b.).
    At temperatures above 65°C cell disruptions occur. If e.g. the waste heat of a CHP unit with temperatures higher than 85°C is used, cell disruption of the biomass in the sewage sludge and therefore unpleasant odors occur.
    Moreover the floor needs to be built in appropriate quality to meet the requirements.
    Result: If the usage of waste heat by means of a floor heating is considered, also air treatment should be taken into account.
  6. When planning the waste heat utilization with air heat exchangers the high temperatures of up to 55°C at the sludge surface and also in the drying hall which can be reached with the SolarTiger® technology have to be kept in mind. It is possible that the air heat exchanger cannot emit the heat as planned under these circumstances.
    The heat exchanger does not permanently run at full efficiency.
  7. During the summer months, the solar radiation is sufficient to dry the sewage sludge with SolarTiger® technology.
    Additional thermal energy cannot be utilized for solar drying during the summer months.


For usage of additional thermal energy, consider:

  • that only 20-25% of it can be used to evaporate water.
  • that it can only be used in cold months or at night.
  • that unpleasant odors may occur.
  • that the requirements for the floor and the quality of the floor covering have to be met.
  • that if all those mentioned points are taken into account, only few cases of waste heat utilization are economic.


Usage of external thermal energy (waste heat) after the solar sewage sludge drying with the SolarTiger® technology.

If thermal energy is available in sufficient quality and quantity, it can be possible, depending on the amount of sludge, to apply a secondary treatment step for sewage sludge, which is dried with SolarTiger® technology. Therein the sewage sludge is tried to securely more than 90% dry matter content and sanitized.


Summary: In order to dimension the thermal energy utilization after solar drying with SolarTiger® technology, please inform us about amount and quality of the available thermal energy. In connection with the sewage sludge amount and the previous solar drying with the SolarTiger® technology we would be glad to develop an according concept for you.