به سوی خانه های کم کربن - یک تجزیه و تحلیل شبیه سازی منبع هوا گرما سیستم های پمپ ساختمان یکپارچه

A comparative transient simulation analysis for domestic buildings with a floor-embedded heating system coupled to a modern air source heat-pump (ASHP) has been carried out using the TRNSYS numerical modeling environment for various UK locations. The effects of heat-pump control during off-peak electricity tariff periods in conjunction with varying building fabric characteristics were analysed and the results show that for the locations investigated, running costs and CO2 emissions were lower for the ASHP platform than for a comparative gas boiler heating system. It was also found that by utilizing the thermal mass of a concrete floor slab or by integrating external insulation, acceptable comfort levels during the heating season were maintained when operating the ASHP solely during off-peak tariff periods. A thinner concrete floor slab containing phase change material (PCM) provided a slightly improved comfort level during winter and also reduced overheating during summer in buildings with a high level of insulation. Finally, when utilising a floor-embedded PCM material, it was found that the thermal properties of the PCM material must be carefully matched with case-specific building fabric thermal performance parameters in order to ensure effective internal environmental control.

The EU faces significant challenges in meeting CO2 emission reduction targets of up to 80% by 2050 compared to 1990 levels. In the UK, domestic dwellings alone are currently responsible for nearly 30% of CO2 emissions. In addition to improved thermal performance of the building fabric, the integration of renewable energy technologies is seen as a key part of the solution required to significantly reduce domestic-sector carbon emissions [1]. With the additional goal to ‘de-carbonise’ the UK's electricity supplies in the medium term, electrically powered building heating solutions (replacing gas, oil or solid fuel powered technologies) are seen as a core strand of the UK's carbon emissions reduction strategy [2]. Within this context, heat pumps are projected to play a major role in domestic-sector heating provision, with air source heat pumps (ASHPs) seen as a particularly attractive option owing to their relatively low cost in comparison with ground-coupled alternatives. Recent research has explored the carbon saving potential of ground-coupled heat pumps, and identified possible barriers to such savings, especially with regards the domestic retrofit sector [3]. Subsequent research [4] evaluated air source heat pump performance within a UK retrofit context, and suggested that carbon emission reduction of up to 12% were feasible compared to a gas boiler system, but with a projected 10% increase in running costs, depending on specific application parameters. Therefore, within such a distributed power generation and distribution framework, the option to employ a degree of demand management by shifting loads from peak to off-peak periods offers an advantage with regards reduced need for extra generation capacity and an increased utilisation of generating plant and hence increased efficiency of generation investment [5]. Heat-pump operational efficiency is often expressed as its coefficient of performance (COP), namely equation(1) View the MathML source Turn MathJax on Therefore, a 6 kWth heat-pump with a COP of 3 will produce 6 kW of heat for every 2 kW of electrical power supplied to the compressor and is therefore approximately 3 times more energy efficient than a 6 kW electric direct heater. For a given electrical power supplied to an ASHP's compressor (Pcompressor), the thermal power (Pthermal) transferred to the heat distribution fluid is normally larger by a factor related to the temperature difference between the heat source and sink. Thus, the COP of an ASHP and its thermal output power decrease when the ambient air temperature decreases or when the heat distribution fluid temperature increases. Since air temperatures are generally lower than those of the ground during the heating season, seasonal COPs for ASHPs are in theory generally lower than for ground source heat-pumps (GSHP) that extract heat from the ground with a more stable temperature [6]. However, ASHP systems are often more convenient and economical to fit, and provided their operational efficiency is optimised, their large market penetration potential means that ASHP technology presents an important route towards significant CO2 reductions [7]. Furthermore, recent field trial data indicates that the performance gap between air and ground source heat pumps in the UK is not as large as previously assumed [8]. Clearly, the ability to utilize a building's thermal mass in order to ‘de-couple’ the electrical input and thermal output of an ASHP offers an opportunity to shift electrical load to off-peak periods, thereby offering electricity system efficiency benefits [7] and [9]. Where electricity suppliers offer tariff options during which electricity is cheaper during off-peak periods, optimising a building's holistic design to facilitate ASHP operation predominantly during off-peak periods also gives this technology a further advantage in term of operating costs. The inclusion of carefully designed phase change materials within the building envelope further enhances these potential benefits [10], [11] and [12]. To de-couple the ASHP electrical and thermal loads, emitted heat is stored in the thermal mass of the building or in dedicated storage tanks and transferred at a later time to the air inside the building or for water heating purposes [9] and [18]. The sensible heat stored in a material is proportional to its thermal capacity, its temperature and its volume. Thus, to increase the sensible energy stored in a material, either (a) the heating distribution temperature; (b) the material volume or (c) the material's specific heat capacity needs to be increased. The former option would not be preferred for an ASHP heating system, since higher outlet temperatures would lead to operation at a lower COP. In addition to increasing the available sensible heat capacity, a further potential option for energy storage is to use fabric-integrated phase change materials (PCMs). These have been investigated previously as a means to store both sensible heat and latent heat when they transition from one state to another (solid–liquid is the preferred transition for building applications). They also provide a stable temperature around their transition temperature when discharging heat leading to potentially improved internal comfort management [10]. The aim of this work is therefore to assess how various control schemes based around off-peak electricity tariff options impact upon the technical and economic performance of an ASHP system for three different dwelling fabric construction methods. Using the dynamic building simulation tool TRNSYS, the objectives of the work included: • For each building configuration, to evaluate ASHP COPs and internal temperature variations for ASHP control based upon a number of currently available off-peak electricity tariff periods; • To investigate performance for three different UK locations in order to evaluate climatic variations; • To assess annual performance in terms of energy consumption, CO2 emissions and running costs for off-peak electricity tariffs, a flat-rate electricity tariff and a gas boiler respectively for each location and building type.

The results presented here are useful in suggesting design approaches for a building with a floor heating system coupled to an ASHP operated during off-peak periods, and suggest that such a strategy can be an attractive option in terms of both CO2 emissions and running costs. Specifically: • The use of an off-peak tariff option helps optimise the balance between enabling acceptable operational efficiency whilst maintaining low running costs; • For a ‘standard performance’ building, either a 20 cm/2300 kg/m3 concrete floor slab with externally insulated fabric construction or a 10 cm concrete slab embedding a PCM melting at 24 °C with walls insulated internally or externally show operational benefits within the conditions and constraints of the model adopted; • For a ‘high performance’ building, a 20 cm concrete floor slab or a 10 cm concrete slab embedding a PCM melting at 22.5 °C is recommended; • The PCM floor slab improves temperature stability during the heating season and reduces the risk of overheating during summer for the high performance building; • The ASHP yearly COP reached 4 for a mild climate and 3.5 for a colder climate with an Economy 10 electricity option. With economy 7, the COP was reduced, which increased the electricity requirement by 11–15% if a night-time set-back control option is not adopted; • In all cases, reduced operating costs and CO2 emissions compared to a design with a condensing gas boiler are indicated; • Increasing the thickness of the floor slab or the use of external insulation can reduce the temperature fluctuations, whilst compared to a thick concrete slab, a thin concrete slab mixed with an encapsulated PCM material melting at 22.5 °C in a highly insulated building displayed a slightly improved temperature stability during the heating season (namely a ca. 0.75 °C minimum temperature increase), but also a greater stability during summer (namely a ca. 1 °C peak temperature reduction), thereby reducing the risk of overheating.