We studied the application of a stratified seasonal hot water storage tank with a heat pump connected to medium-, low- and very-low-temperature space heat emissions for a single-family house in Stockholm, Sweden. Our aim was to investigate the influence of heat emission design temperature on the efficiency and design parameters of seasonal storage in terms of collector area, the ratio of storage volume to collector area (RVA), and the ratio of height to diameter of storage tank. For this purpose, we developed a mathematical model in MATLAB to predict hourly heat demand in the building, heat loss from the storage tank, solar collector heat production, and heat support by heat pump as a backup system when needed. In total, 108 cases were simulated with RVAs that ranged from 2 to 5 (m3 m−2), collector areas of 30, 40, and 50 (m2), height-to-diameter-of-storage-tank ratios of 1.0, 1.5, and 2.0 (m m−1), and various heat emissions with design supply/return temperatures of 35/30 as very-low-, 45/35 as low-, and 55/45 (°C) as medium-temperature heat emission. In order to find the best combination based on heat emission, we considered the efficiency of the system in terms of the heat pump work considering coefficient of performance (COP) of the heat pump and solar fraction. Our results showed that, for all types of heat emission a storage-volume-to-collector area ratio of 5 m3 m−2, with a collector area of 50 m2, and a height-to-diameter ratio of 1.0 m m−1 were needed in order to provide the maximum efficiency. Results indicated that for very-low-temperature heat emission the heat pump work was less than half of that of the medium-temperature heat emission. This was due to 7% higher solar fraction and 14% higher COP of heat pump connected to very-low-temperature heat emission compared to medium-temperature heat emission.
In 2013, the building sector consumed approximately 39 % of the total final energy use in Sweden. Energy used for heating and hot water was responsible for approximately 60 % of the total energy consumption in the building sector. Therefore, energy-efficient and renewable-based heating and ventilation systems have high potential for energy savings. The potentials studied in this thesis include the combination of a low-temperature heat emitter (supply temperature below 45 °C) with heat pump and/or seasonal thermal energy storage, and variable air volume ventilation system. The main aim of this thesis was to evaluate energy savings and indoor air quality when those energy-efficient and sustainable heating and ventilation systems were implemented in buildings. For this purpose, on-site measurements, lab tests, analytical models, and building energy simulation tool IDA Indoor Climate and Energy 4 were used.
Annual on-site measurements for five new two-family houses with low- and very-low-temperature heat emitters connected to an exhaust air heat pump showed that between 45–51 kWh∙m-2 energy was used to produce and transport supply water for space heating and domestic hot water. Statistical data showed that these values are 39–46 % lower compared to the energy requirement for the same usage which is, 84 kWh∙m-2) in an average Swedish new single- and two-family house.
Annual on-site measurements for five new two-family houses with low- and very-low-temperature heat emitters connected to an exhaust air heat pump showed that between 45–51 kWh∙m-2 energy was used to produce and transport supply water for space heating and domestic hot water. Statistical data showed that these values are 39–46 % lower compared to the energy requirement for the same usage (which is, 84 kWh∙m-2) in an average Swedish new single- and two-family house.
In order to compare the energy performance of very-low- and low-temperature heat emitters with medium-temperature heat emitters under the same condition, lab tests were conducted in a climate chamber facility at Technical University of Denmark (DTU). To cover the heat demand of 20 W·m-2 by active heating, measurements showed that the required supply water temperatures were 45 ºC for the conventional radiator, 33 ºC in ventilation radiator and 30 ºC in floor heating. This 12–15 ºC temperature reduction with ventilation radiator and floor heating resulted in 17–22 % savings in energy consumption compared to a reference case with conventional radiator.
Reducing the supply temperature to the building’s heating system allows using more renewable and low-quality heat sources. In this thesis, the application of seasonal thermal energy storage in combination with heat pump in a building with very-low-, low-, and medium-temperature heat emitters was investigated. Analytical model showed that using a 250 m3 hot water seasonal storage tank connected to a 50 m2 solar collector and a heat pump resulted in 85–92 % of the total heat demand being covered by solar energy.
In addition to the heating system, this thesis also looked at ventilation system in terms of implementing variable (low) air volume ventilation instead of a constant (high) flow in new and retrofitted old buildings. The analytical model showed that, for new buildings with high volatile organic compound concentration during initial years of construction, decreasing the ventilation rate to 0.1 L·s-1·m-2 during the entire un-occupancy period (from 8:00–18:00) creates unacceptable indoor air quality when home is occupied at 18:00. So, in order to create acceptable indoor air quality when the occupants come home, a return to the normal ventilation requirements was suggested to take place two hours before the home was occupied. This eight- hour ventilation reduction produced savings of 20 % for ventilation heating and 30 % for electricity consumption by ventilation fan.
In addition, the influence of different ventilation levels on indoor air quality and energy savings was studied experimentally and analytically in a single- family house occupied by two adults and one infant. Carbon dioxide (CO2) concentration as an indicator of indoor air quality was considered in order to find appropriate ventilation rates. Measurements showed that, with an 0.20 L∙s-1∙m-2 ventilation rate, the CO2 level was always below 950 ppm, which shows that this level is sufficient for the reference building (CO2 lower than 1000 ppm is acceptable). Calculations showed that low ventilation rates of 0.20 L∙s1∙m-2 caused 43 % savings of the combined energy consumption for ventilation fan and ventilation heating compared to the cases with 0.35 L∙s-1∙m-2 as a normal ventilation rate recommended by BBR (Swedish Building Regulations).