تجزیه و تحلیل عملکرد سیستم های گرمایش برای خانه های با انرژی پایین

The residential sector is responsible for more than 35% of the final energy consumption in the European Union, and is increasingly constrained by thermal regulations, resulting in a significant rise of new efficient heating technologies. This paper presents a comparison of the energetic and environmental performances of six heating systems installed in a low energy house: a gas-fired condensing boiler, a wood pellet boiler, a micro-combined heat and power (MCHP), an air-to-water electric heat pump, an air-to-water gas absorption heat pump and an exhaust air-to-air electric heat pump. The comparison is made with respect to the annual primary energy consumption and the annual greenhouse gas (GHG) emissions, and carried out under various climates and electricity generation mixes. The results indicate that, based on an ideal sizing, the MCHP and the absorption heat pump achieve the highest energy performances. However, both technologies suffer from small size unavailability, leading to significant oversizing which impacts their performances. Based on current size availability, the air-to-water electric heat pump benefits from the previous systems oversizing and thus appears as the most efficient technology. However, current sizing practice also causes significant oversizing which impacts the performances of thermodynamic systems. Without significant sizing practice improvements, the air-to-water electric heat pump merit decreases in favor of the MCHP. In terms of environmental impact, the wood boiler causes the lowest GHG emissions, whatever the electricity generation mix considered.

Residential sector contributes to more than 35% of the final energy consumption, and to a large part of the GHG emissions in the EU, leading to a significant impact on the environment. In 2010, the EU adopted a Directive on energy performance of buildings [1], aiming for major cut in building sector annual energy consumption. Under this Directive, Member States must establish and apply minimum energy performance requirements for new and existing buildings. In France, this requirement is set to 50 kWh/m2 of primary energy per year for new buildings including heating, cooling, auxiliaries (pumps and fans), domestic hot water and lighting, with a slight variation depending on local climate, and 80 kWh/m2 per year for retrofit buildings. In order to meet these new standards, building insulation, air renewal management, heat production as well as thermal regulation should be carefully addressed. Peeters et al. [2] evaluated the impact of building insulation quality and emitters control strategy on overall heating efficiency. They showed that heating efficiency tends to decrease with building insulation improvement, since heating system oversizing becomes more important. The study also showed the interest of using a modulating boiler, an outdoor air temperature compensation and thermostatic radiator valves. Calvino et al. [3] revealed that the application of a new PID-fuzzy controller instead of a classic on-off controller for indoor thermal comfort could result in energy savings and lower deviation from temperature set point. Badran et al. [4] performed a comparative study of continuous versus intermittent heating in residential buildings. For buildings with high insulation level, continuous operation of the heating system at low water temperature becomes more economical than intermittent operation at high temperature when the operation time per day exceeds 14 h. El Fouih et al. [5] carried out a comparison of the energy performance of three ventilation systems in several types of low-energy buildings. In the case of a residential house, humidity controlled ventilation appears to be the most efficient ventilation system for warm and moderate climates. For cold climates, heat recovery ventilation can become the most efficient system if high efficiency heat exchanger and low specific fan power are used. Energy performance and environmental impact comparison of heating systems is a major topic that has already led to numerous publications. Cabrol and Rowley [6] estimated the CO2 emission savings achieved by the use of an air source heat pump instead of a conventional gas boiler. Results vary between 26% and 36%, depending on the building and the climate considered. Kelly and Cockroft [7] also evaluated these savings in the case of heat pumps retrofitted into dwellings. Simulations validated by trial data indicated that heat pumps produce 12% less GHG emissions than equivalent condensing gas boilers. Yang et al. [8] carried out a comparison of hot water and forced air heating systems in terms of annual exergy consumption, life-cycle GHG emissions and life-cycle energy use. Results indicated that hot water heating systems using either electricity or natural gas and coupled with heat recovery ventilation have the lowest annual exergy consumption and life-cycle energy consumption. However, life-cycle GHG emissions depend largely upon electricity mix. Same conclusions have been drawn by Shah et al. [9] while performing a life cycle assessment of an air-to-air heat pump compared to a conventional gas furnace and gas boiler coupled with an air conditioning system. Eventually Dorer and Weber [10] assessed the energy performance and CO2 emissions of several micro-cogeneration systems and compared them to conventional heat pumps and boilers. The heat pump appeared to offer the maximal CO2 emissions savings compared to a gas boiler, whereas micro-cogeneration systems can achieve maximal primary energy consumption reductions, up to 34%. Although these studies have focused on the comparison of heating systems through energetic and environmental criteria, the types of heating systems compared was mainly limited to gas boilers and electric heat pumps, and did not reflect the large range of heating systems currently available for residential heating. While boilers have long constituted the vast majority of the heating systems used in the residential sector, increased regulatory constraints have recently resulted in a significant rise of new efficient heating technologies. Although some studies focus on one of these emerging technologies, none of them has taken into account all these technologies in either an energetic or an environmental analysis. Such study is however essential for addressing their relative merits and for understanding future market development. Furthermore, the sizing issue is rarely discussed. This is of particular interest for low energy buildings as all heating products are not available in the small power range, which may lead to important system oversizing and change the order of merit of the different heating systems. This study aims to evaluate the energetic and environmental performances of six heating systems installed in energy-efficient residential buildings. The systems studied are: (a) gas-fired condensing boiler, (b) wood pellet boiler, (c) micro-CHP, (d) air-to-water electric heat pump, (e) air-to-water gas absorption heat pump, and (f) exhaust air-to-air electric heat pump. The study intends to compare these six heating systems in terms of annual primary energy consumption as well as annual GHG emissions according to different European climates, while accounting for the effect of sizing and the limit of small size availability.

In this paper, a methodology for assessing the performances of six different heating systems in terms of primary energy consumption and GHG emissions was developed and applied to a typical low energy house under various European climates and considering two different energy mixes for electricity generation. The aim of the study was to compare the relative merit of the technologies, while assessing the effects of small size availability, oversizing and heating strategy. Based on the hypothesis of ideal small size availability, the air-to-water absorption heat pump and the MCHP reach the best performances for a very large range of climates. However, these technologies are new for the residential sector and suffer from limited availability in terms of size. The oversizing led by their limited size availability reduces their performances to such a point that the air-to-water electric heat pump has the lowest energy consumption for almost all climates since their current size availability is the most adequate. Moreover, the large oversizing due to current sizing practices impacts heat pumps more than other technologies. The relative merit of the air-to-water electric and absorption heat pumps is thus highly dependent upon good sizing practice, and the availability of small absorption heat pumps is only useful if this practice improves. Eventually, the MCHP benefits from current bad practice, and would perform highest performances if no significant improvement is proposed. Contrary to the sizing, the heating strategy does not have a significant impact on either the primary energy consumption or the relative merit of each technology. However, a continuous heating strategy leads to heating system size reduction, thereby reducing their acquisition cost. This study focuses on evaluating heating systems through energetic and environmental criteria. Potential future research could balance these comparisons by a cost analysis. The energy performance of a heating system is essential to fit the regulatory requirements. However, the heating system final selection is usually based on economic criteria. From this standpoint, electrical heating, whose energy performance is significantly lower than those of the systems considered in this study, benefits from a low investment cost and may be competitive, especially in regions with low cost electricity generation. In addition, further efforts should be undertaken in the modeling of exhaust air-to-air electric heat pumps, thus enabling performance assessment of bigger systems.