Evaluation of new active technology for low-energy houses

Detta är en avhandling från Energy and Building Design, Lund University

Sammanfattning: Using energy at low-quality levels opens up new possibilities for low-energy houses. Low-quality energy can be heat at a temperature that is close to that of its surrounding, and can be used, for example, to pre-heat ventilation air or domestic hot water. Pre-heating the incoming outdoor air reduces the need to heat ventilation and reduces the need for high-quality energy such as electricity or heat from a fire. This thesis investigates two such possible energy utilizations, the PV/T solar window and the hybrid ventilation system. They are very different in how they reduce the need for auxiliary energy in buildings, and they cover different fields of low-energy building technique. However, what they have in common is the concept of low-quality energy. The solar window produces both electricity and hot water. What the photovoltaic cells cannot utilize at the high-quality energy level is instead used to produce hot water. The hybrid ventilation system pre-heats the incoming ventilation air in the heat recovery system, thereby lowering the need for high-quality energy. The PV/T solar window comprises PV cells laminated on solar absorbers placed in a window behind the glazing. To reduce the costs of solar electricity, tiltable refl ectors were included in the design to concentrate solar radiation onto the solar cells. The refl ectors enable control of the amount of solar radiation transmitted into the building. The insulated refl ectors also reduce thermal losses through the window. The effects on the light distribution and the architectural implications are discussed in earlier studies (Fieber, 2005; Fieber et al., 2003; Fieber, Nilsson, & Karlsson, 2004) together with effects on the building when different strategies for controlling the reflectors are used. Long-term measurements were taken of the thermal- and electrical energy output from the solar window. A model was developed to simulate the electricity and hot water production, and the model was calibrated against the measured values from a prototype solar window installed in a laboratory and against a solar window built into a single-family building. The results from the simulation showed that the solar window produces about 35% more electrical energy per unit cell area than a vertical flat PV module. However, PV cells placed on the roof of the building would produce approximately 17% more electricity per unit cell area than the solar window. The simulations carried out on system level showed that installing a 16 m² solar window (glazed area) in a single-family building reduces the annual heating need by approximately 600 kWh. However, if the absorbers (5.06 m²) and PV cells (4 m²) from the solar window are installed separately on the roof instead of in the window, the annual heating need is reduced by a further 1100 kWh. A water-to-air heat exchanger was developed for use in naturally ventilated buildings. This requires that the pressure drop of the air is kept close to zero. The heat exchanger comprises solar collector absorbers soldered onto a manifold. Basic heat transfer equations were used in order to optimize the dimensions of the heat exchanger in terms of heat transfer and pressure drop. A laboratory measurement showed the temperature heat recovery rate to be 80% at component level. At the same time the pressure drop was 1 Pa for the designed air flow rate. System simulations were then carried out in order to investigate the impact for a building equipped with natural/hybrid ventilation with heat recovery. A brine-based heat recovery system enables the utilization of other energy sources such as ground collectors or waste water heat recovery units. A waste water heat recovery system was built into a single-family house, and was designed to supply energy to both domestic hot water and the ventilation system. The simulations showed that a typical single-family house can reduce the heating need by approximately 600-800 kWh annually, i.e. roughly 25% of the annual need for hot water, with waste water heat recovery. The simulations showed that using ground collectors for the ventilation system has limited effects on the heating need, so the main benefit is limited to lowering the risk of frost on the heat exchanger surface. The overall conclusion from an energy perspective is that the solar window performs poorly compared to standard solar energy components. The hybrid ventilation system with the developed heat exchangers has the potential to be an interesting ventilation system when building low-energy houses or when renovating residential buildings to improve energy effi ciency.