Investigation of a low-energy thermal energy recovery system for passive ventilation applications

Abstract

Passive ventilation systems such as wind towers can provide fresh air to buildings with no energy demand, however, are often closed throughout the winter of cooler climates due to the introduction of untreated fresh air into the indoor environment which can result in increased building energy demand. Wind towers were originally used throughout The Middle East for centuries to provide passive cooling in hot, arid climates. They have since been modified for European climates, altering the shape of the external envelope, the number of sides, and the use of louvres to channel the flow of air through the system. Various forms of heat recovery have been explored within passive ventilation, however their performance is limited by the small temperature differences between the exhaust and inlet air channels and the allowable pressure drop over the heat recovery device. Therefore, it is desirable to introduce a heat recovery system capable of providing a greater degree of heating and cooling than existing systems, with greater control afforded to the user, whilst incurring a low pressure drop to maintain ventilation rates through the system. As such, this research aimed to combine heat recovery within a passive ventilation system to create a low-energy heating, cooling, and thermal energy recovery system.

The study followed an iterative design approach, using two separate physical tests and corresponding numerical models. First, the impact of an increasing number of horizontal pipe layers through a multi-sided wind tower was explored, measuring the volumetric flow rate into the room below as well as the potential direct heat recovery from the exhaust to inlet air channels. A full-scale wind tunnel test was conducted in an environmental chamber at the Building Research Establishment site in the UK, with the results used to validate a corresponding numerical model developed through ANSYS Fluent under steady state conditions. Once validated, simulations were conducted under increasing wind speeds and under differing inlet and outlet air temperatures to understand system performance.

The results of the wind tunnel test and numerical modelling were used to guide design decisions for the integration of a water-based thermal loop to extract and deliver thermal energy from and to the system. Once the design of the system was settled, a field trial of the proposed system was conducted at an office site in Sheffield, UK. The ability of the system to cool fresh air was recorded over several days, with the results used to validate a second numerical model through a transient simulation, again developed through ANSYS Fluent. Once validated simulations further steady state simulations were conducted, varying the wind speed, air temperatures, water temperatures, and water velocities, measuring the ability of the system to provide heating and cooling to fresh air whilst recovering waste thermal energy.

For the wind tunnel test, heat pipes were installed within a wind tower to facilitate the direct recovery of waste heat from the exhaust to inlet air channels. The wind tower and heat pipe system maintained a volumetric flow rate of 0.1 m3/s at an inlet velocity of 1 m/s. The fresh air temperature was raised by up to 2.8 °C, with the amount of heating increasing as the inlet velocity fell and the temperature difference between the exhaust and inlet channels rose.

For the field trial, a run-around heat exchanger was integrated into a wind tower to recover waste thermal energy and eventually link the system with seasonal thermal energy storage. Validating the numerical model using experimental data, simulations revealed the potential of the system to provide fresh air at 0.09 m3/s when the inlet velocity was 1 m/s, marginally lower than the wind tunnel test owing to the increased wind tower height. A maximum pressure drop over the heat exchanger of 2.07 Pa was recorded at a wind speed of 5 m/s. The fresh air temperature was decreased and increased by up to 4.36 and 4.14 °C respectively due to cooling and heating provided through the heat exchanger, with the amount of heating or cooling again increasing with decreasing wind speed. Under the same boundary conditions, an increase or decrease of 6.15 °C in water temperature through the heat exchanger was reported, when providing heating and cooling. The change in water temperature remained the same under heating and cooling conditions due to the variations in inlet and exhaust air temperature through the wind tower inlet and outlets. Considering the system was intended to recover thermal energy for storage through a form of seasonal thermal storage, a maximum of 2 kW of sensible energy was transferred between the fluids.

Adding fins to the heat exchanger reduced the volumetric flow rate into the room but improved the heat transfer between the fluids. The performance of the standard heat exchanger was evaluated through the efficiency and overall heat transfer coefficient, achieving a maximum efficiency of 30% and a heat transfer coefficient of 5.1 W/m2 – K.