Wind- and stack-assisted mechanical ventilation with heat recovery and night cooling

Abstract

The dual-sided issue of indoor environment and energy consumption have become increasingly important in building design. One possible solution is to ventilate by passive means, such as by stack eect and wind pressure, but this requires the development of new concepts and components. Here we have presented the outline of a heat recovery concept suitable for stack and wind-assisted mechanical ventilation systems with total system pressure losses of 74Pa. The heat recovery concept is based on two air-to-water exchangers connected by a liquid loop powered by a pump. The core element of the concept, a prototype of a heat exchanger, was developed based on design criteria about pressure drop, eciency and production concerns. The exchanger is based on banks of plastic tubing cris-crossing the air flow, thus creating approximate counter flow between air and water. Round PE plastic tubing is used. The tubing is commonly used for water-based floor-heating systems. Oval or even wing shaped tubes may have better heat transfer and lower drag coecient, but round tubes require less meticulous production procedures. The tubing used here is mass-produced, cheap, and flexible but the current design does require many fittings. Multiple design proposals were modeled and investigated to determine the optimal solution with respect to pressure drop, heat transfer, and production feasibility. Software models were developed to simulate the temperature distribution within the tube bank. The final design involves two parallel tubes cris-crossing the air fixated in a `radiator' by tube bending brackets and spacers. The radiators are stacked in a staggered grid to force a more tortuous air flow and generate as much heat transfer as possible. The performance was confirmed by comparing the pressure drop and heat transfer measured on a section with numerical fluid calculations and literature sources. The measurements and calculations agree reasonably well. A full scale implementation was achieved in a part of building 118 at Dept. of Civil Eng., Tech. Univ. of Denmark. In this building the system supplies offices and two teaching rooms with fresh air. The mean specific fan power (SFP, in Danish: SEL-værdi) is measured to be approx. 240 J=m3. The SFP of the entire system including fans, pump and controls is measured to be approx. 600-700 J=m3. The total heat recovery efficiency of the system was measured to be 63%. Night cooling is implemented in the CTS-system, which means that the building is cooled at night in warm summer periods based on the temperature sensor in a critical room. With the extremely low fan power consumption, the cooling is practically free.