Dynamic simulation of the reversible heat pumps performances in non residential buildings at different latitudes in Europe
In this paper a comparison between separated heating and cooling vs. reversible heat pumps based configurations in the HVAC equipment of a significant non residential building, is carried out over an entire reference year. Simulation is performed with hourly time steps in two European cities (Berlin and Rome) taking into account latitude related weather conditions, envelope properties, whole building response when air conditioning is switched on or switched off. A sensitivity analysis is carried out considering different solutions for facades and fenestration properties, and lamps efficiency. The comparison reveals an higher sensitivity of the results to climate severity, windows shading, and inner lighting levels. The discussion of the results leads to assess the competitiveness of reversible heat pumps in terms of energy demand, as a function of the considered options. Introduction Reversible heat pumps are increasingly considered in building's HVAC applications across all Europe as a remarkable driver of decarbonization; so much that the standard for the energy performance of electrical heat pumps published in March 2016 (European Committee for Standardization, 2016) recently implemented the Ecodesign Directive 2009/125/EC (European Parliament 2009) stating new performances evaluation criteria, assuming the Strasbourg's city climate as a reference. Moreover, the use of halogen incandescent lamps was recently inhibited in the EU (in Italy since September 2018) thus widely implementing lighting solutions based on LEDs technology. Nevertheless the European climate has a wide range of variability (Spena, 2010) mainly due to latitude's span and different distances from the sea. In addition, as latitudes rise and internal gains decrease as a consequence of higher energy efficiency, generally the building's heating loads grow, while cooling and electricity needs tend to decrease. As widely recognized in the last decade (Ashrae, 2017; Reddy, 2006; Rodriguez, 2013; Brouns, 2016) the actual behavior of a nonresidential building (Pessenlehner, 2003) can be predicted only by a dynamic simulation (Lam, 1996; Shiel, 2018) Present paper investigates the sensitivity of the results when using reversible heat pumps as central heating and cooling equipment, in two different and significant Europe's climates, in order to explore to what extent the competitiveness of reversible heat pumps can differ through the EU. Of this topic, not enough stressed in the literature, still remains poor the awareness in the European standards. The first part of this work, mainly concerning wholebuilding dynamic simulation, will mainly deepen on the effects of severe winter conditions and of fenestration features on the energy demand of the considered building. The second part, consisting in the sensitivity analysis, will deepen on how its equipment arrangement, especially in case of reversible heat pumps, could be optimized depending on the site. Methods Source code Building energy simulation programs, representing different degrees of modeling complexity, have been intensively tested in the last decade (Judkoff, 2011 and 2014) But all building models are simplification of reality, and a complex balance among level of modeling, time step granularity, and scope of the simulation is always needed (ASHRAE, 2017) For the purpose of the present study, which is essentially of comparative nature, a proprietary SW code created in the '80s (Spena, 1984) and over time improved and devoted to specific purposes, was dedicated and implemented. Weather data, set point temperature levels, building envelope and fabric properties, equipment performances are described partly by deterministic (Spena, 2010) partly by stochastic models (Spena, 1997) using algorithms and source code routines. The source code structure The simulation code was applied to the whole building system, to obtain trends of energy demand over time to be used as the basis for comparisons and sensitivity analysis. Two locations representative of respectively a continental (Berlin, lat. 52,3degN) and a Mediterranean (Rome, lat. 41,5degN) climatic condition at nearly equal longitude (12,3deg vs. 13,2deg) were compared. The following issues were considered. Glazing: single glass, low emissivity double glass, low emissivity triple glass, low emissivity Fenestration: no shading plastic roller filled with insulating polymers ________________________________________________________________________________________________ ________________________________________________________________________________________________ Proceedings of the 16th IBPSA Conference Rome, Italy, Sept. 2 4, 2019 2071 https:/doi.org/10.26868/25222708.2019.211147 The code structure is a main routine supported by four dedicated subroutines operating iteratively (Spena, 1984; Spena, 2017) As a first approach, to simplify the procedure, the following counters are used: one reference day per month four surface exposures building usage range from 7:00 to 20:00 hrs. Daily results are extended prior monthly and then yearly in terms of total heating, cooling and electrical energy demand. The reference fabric is an existing large nonresidential building situated in an urban area of Rome (see Figure 1) weekly used from Monday to Friday. It may be noticed that its common architecture appears representative enough for the aim of the present study: opposite feature departures will be explored by the sensitivity analysis. Figure 1: Building satellite view. External conditions Thermal flows through the walls were obtained from Fourier's general equation, also taking into account the inward flowing fraction of absorbed solar radiation by means of the so called sol air temperature (ASHRAE, 2017) A generally stabilized periodic regime was simulated (Tabunschikov, 1993; Spena, 2017) as follows: q(t) U (Tsa Ti) U s (Tsa(t ph) Tsa (1) where: t time (hrs) U overall heat transfer coefficient W m2*K Tsa sol air temperature (degC) Ti indoor air temperature (degC) s heat flow damping (dimensionless) ph phase lag (hrs) Correction factors were introduced in (1) to properly consider either the effects of solar radiation on walls, either multi layer walls together with their surface convective phenomena. Outdoor weather data are taken from reference years elsewhere described (Spena, 2017; http:/www.eurometeo.com) Solar radiation Heat loads due to solar radiation on the building envelope varies according to the sun vs. surfaces relative postioning. We considered as independent variables: i) location, i.e. geographical coordinates; ii) time; iii) cloud coverage; iv) facade exposures. The ASHRAE atmosphere model (ASHRAE, 2017) was implemented with a Monte Carlo based algorithm (Spena, 1997) able to generate the components of the total solar radiation. The building model The building of Figure 1 has an aspect ratio (surface area over volume) of roughly 0,3. It consists of 8 floors for an overall height of about 27 m. The overall enveloping area amounts to 12.800 m. Office rooms cover the 64% of available floor area; the remaining 36% is connective space. Table 1 shows the envelope wall properties. Table 1: Walls properties. U W m2*K Thickness (m) External walls 0,302 0,45 Roof 0,279 0,36 Ground floor 0.360 0,52 Building fenestration varies as a function of the different exposures, its (window area/overall facade area) ratio values ranging from a minimum of 0,20 to a maximum of 0,48. Glazing properties are reported in Table 2, where SHGC is the solar heat gain coefficient (ASHRAE, 2017) Table 2: Glazing properties. U W m2*K Transmittance SHGC Single glass 3,023 0,8 0,19 Double glass 1,511 0,64 0,15 Triple glass 1,279 0,51 0,12 In the base configuration, windows are double glazed. In case of window shaded configuration, an additional thermal resistance of 0,192 K*m2 W was considered. Facilities HVAC Full air conditioning within nearly all the building space is provided by an air controlled ventilation fan coilssystem. A set of AHU are devoted to balance latent heat internal loads and to supply the standard required ACR; while fan coils units balance sensible heat loads. During winter season primary air is pre heated, then humidified and post heated; during summer season primary air is cooled and dehumidified, and post heated. Indoor setpoint temperatures and ventilation rates are assumed according to the standards respectively FprEN 167981:2018 (ECS, 2018) and ISO 17772 1:2017. ACR V m3 h (2) Lighting According to the standard UNI EN 12464, indoor lighting levels are assumed (ECS, 2011; Fontoynont, 1999) for offices and connecting spaces at average values of respectively 400 lx, and 50 lx. Natural lighting contribution was estimated on the basis of the Average ________________________________________________________________________________________________ ________________________________________________________________________________________________ Proceedings of the 16th IBPSA Conference Rome, Italy, Sept. 2 4, 2019 2072 Daylight Factor and of the fenestration actual area. Either LEDs lighting technologies based on lamps with efficiency of 70 lm/W, and halogen incandescent lamps with efficiency of 22 lm/W were simulated and compared. Central heating and cooling equipment The following configurations are compared: 1. Heating and cooling separately provided by fuel fired furnaces, and by electrical chillers (FFC) 2. Both heating and cooling provided by reversible heat pumps (RHP) The reversible heat pumps performance (3) was splitted over two coefficients, depending one on external temperature (Figures 2 and 3) and the other on load factor as follows (Figures 4 and 5) COP COP(Tout) x (3)