PCSP’s Diagonal Tie Connectors Thermal Bridges Impact on Energy Performance and Operational Cost: Case Study of a High-Rise Residential Building in Estonia

This study analyses the effect of air circulation around diagonal tie connectors in precast sandwich panels on heating energy demand, energy performance value and heating costs of a sample residential building. Dynamic simulations were performed using 4 different climatic boundary conditions: Estonian test reference year, Estonian 48-year weather dataset as well as data from Eastern Germany and Northern Finland. The results show that the effect of the thermal bridge is most noticeable in total room heating energy demand (increase of 10.3%), while the influence on energy performance value was 1.1%. The relative increase of total room heating energy demand was similar (7.0-10.3%) in all studied climatic regions. Abbreviations and symbols DTC diagonal tie connector MEP mechanical, electrical and plumbing system nZEB nearly zero energy building PCSP precast concrete sandwich panel A net floor area (m2) EPV energy performance value [kWh/(m2×a)] Ψ thermal bridge conductivity [W/(m×K)] T temperature (K) U thermal transmittance [W/(m2×K)] te external air temperature (°C) ti indoor air temperature (°C) ts supply air temperature (°C)


Introduction
The construction industry includes a variety of building components. The demand for high quality, faster and environmentally friendlier construction and energy savings has maintained the need of prefabrication methods. Prefabricating building parts as concrete structures should not be defined only as parts of the structure precast in specialized plants to be assembled afterwards on site. For optimum exploit, precast concrete structure should be designed according to the specific rules from the very outset [1]. Building design involves progressive methods and components to reach high-level building energy performance. Using PCSPs with rigid thermal insulation materials is one option to reach proper building envelope goal of optimal heat transfer. Investigating the overall thermal performance caused by diagonal tie joints, Klõšeiko et al. [2] found that air cavities around the connectors increased the thermal conductivity of the sample wall up to 55% (Fig.  1). If no additional insulation around diagonal tie connectors (DTC) is installed, natural air convection occurs in cavities between two insulation sections. However, to evaluate the magnitude of the effect, this paper focuses on delivered room unit heating energy (excluding ventilation heating and domestic hot water) and energy performance value (EPV) analysis with regional climate impact comparison on the example of a typical high-rise residential building in Tallinn.  Several studies are available in the field of apartment building energy performance analysis in Estonia. Hamburg et al. [3], [4] claimed measured heating energy to be higher mainly because of higher indoor air temperature (ti), air handling unit supply air temperature (ts), using windows for air exchange and higher ventilation rates. During winter months, mean ti +23°C in apartments was measured. cover thoroughly the requirements, methods and instructions to calculate energy efficiency for new and significantly reconstructed buildings. Extra cost and economic benefit analysis of energy efficiency conducted by Pikas et al. [10], [11] indicate economic benefits for both small scale and national level. Heating distribution and emission efficiency in residential buildings in Estonia was investigated by Maivel and Kurnitski [12], ventilation performance was well analysed by Mikola [13]. Simson et al. [14] investigated the heat loss effect of calculating methods related to building infiltration in Estonian climate conditions with software IDA ICE [15]. It was found, that the highest risk of under dimensioning is with the 1-storey building as the stack effect with multi storey buildings is more accurate.
The novelty of this study is to expand previous work [2] supported by local residential building research, attainments and construction practice. The target questions in this paper are aimed to determine the impact of the total room heating energy extra losses and operational costs through PCSP DTC Ψ and to assess the impact on the building EPV.

Methods
This section describes the methodical steps ( Fig. 3) of the paper in addition to the PCSP' diagonal tie connector Ψ impact specified by Klõšeiko et al. [2]. Simulation input data for the analysis includes reference object and typical floor plan information described and shown in Fig. 2 and Fig. 4. Tables 1-4 describe simulation input used for building envelope, MEP system and internal gains parameters. Description of the macro used in IDA ICE software [16], based on the Klõšeiko et al [2] Equation 1, is seen on Fig. 5. Simulations section defines the temperature setpoint and climate scenarios (Fig. 6) for regional impact analysis on total room heating energy loss. Different external air temperature curves of Estonian test reference year [17], ASHRAE Fundamentals 2013 weather design data for Berlin, Germany and Rovaniemi, Finland are presented in Fig.  7. Envelope and MEP system parameters for the EPV analysis are shown in Table 5 and Table 6.

Simulation input data
Depending on the specific building parameters, number of similar elements are varied. However, the prefabrication process efficiency depends on the amount of the similar parameters for the product. Therefore, a typical PCSP usage is a high-rise residential building external wall perimeter. The reference building is a modern 14-storey apartment building, built between 2016 and 2019, located in Tallinn, Estonia (Fig. 2). We have analysed a typical floor layout of the building to quantify the effects of PCSP-s.   The building model was composed and the calculations were conducted using well validated [15] building simulation software IDA ICE, version 4.8 SP1, EQUA Simulation AB, Stockholm, Sweden [16]. The typical floor contains 7 zones with 6 apartments with floor areas from 45 to 70 m² (Fig. 4). Reference building envelope, thermal bridges and infiltration parameters are listed in Table 1   Reference building MEP systems, including heating and ventilation, were based on current regulations [5], [9] listed in Table 3. No cooling system was in the reference building design or used in this paper. To assess actual total room heating energy extra loss and calculate EPV [9], internal heat gains such as occupants, lighting and appliances must be taken into account in simulations. Internal heat gain values and time schedules are presented in Table 4.
where ΔT is the temperature difference between ti and te.
The macro scheme for the extra room heating energy loss calculation for each zone in IDA ICE is shown in Fig. 5. These 7 macros are placed as one overall macro into the IDA ICE "Standard Plant" and the extra heat losses are merged with base model total room heating energy. The extra heat loss described in Fig. 5, includes "TAmb" as te, "Polynomal" as the function of Equation 1, "LinkRef" as the wall surface area m² per each external wall and " Step" as the average step between DTCs. Although the default step for DTCs is 0.6 m, an average step length was calculated for each apartment and the hallway and entered separately into macros.

Simulations
Estonian local climate data used in this study was based on Estonian test reference year [17]. Long-term data based on climate from Tõravere, Tartu weather station from years 1970 to 2018, was used to assess the amplitude of the deviation between different decades. Reference building with the same input parameters was also simulated in more varied climate regions. For colder weather Rovaniemi, Finland and for warmer weather Berlin, Germany were used according to ASHRAE Fundamentals 2013 weather design data [16]. Thus, simulations with 2 different ti and ts variables using 4 different climatic boundary conditions were conducted (Fig. 6). External air temperature duration curves for EstonianTRY, Eastern Germany and Northern Finland are shown on Fig. 7.   Fig. 6. Temperature setpoint and climate scenarios. Fig. 7. External air temperature duration curves for regional impact analysis. In addition to assessing impact on EPV in comparison of reference building base model and DTC extra loss, thermal energy sources and a sample of envelope and MEP parameter variations were analysed. Base model thermal energy source as an effective district heating is described in Table 3. For the purposes of the current regulation [5], efficient district heating (or cooling) is described as: "A district heating system that uses at least 50% renewable energy, 50% waste heat, 75% cogeneration or 50% combination of such energy and heat". Other thermal energy source options with differences between reference building base model, including default district heating, pellet boiler, oil and gas condensing boiler, solid fuel and peat boiler, air-towater and geothermal heat pumps, are listed in Table 5. The sample of envelope and MEP parameter variations was analysed in this study. Ventilation system options included comparison of electrical ventilation heating for central air handling unit and rotary heat exchanger comparison instead of plate for apartmentbased air handling unit. The current regulation [5] allows 0.42 l/(s×m²) air exchange rate instead of 0.5 if air exchange rate can be controlled in the boundaries of the single apartment. Manner of room heating type was changed from underfloor heating system to radiator. Window thermal transmittance was reduced by 9%. Infiltration was lowered to least allowed value by regulation, as if it is planned to measure the air leakage during the construction of the building [9]. Electrical domestic hot water system and 25% lower lighting installed power per m² options were analysed. The differences between reference building base model and a sample of envelope and MEP parameters analysed for the comparison are listed in Table 6.
District heating is used as a basis for operational cost analysis. The district heating price used in this paper is 58.8 €/MWh including (49 €/MWh without) VAT [18].

Results and discussion
In this paper, room heating energy extra loss and operational cost, regional impact on room heating energy loss and impact on energy performance, due to the PCSP's DTC extra thermal bridge was analysed. General DTC room heating energy extra loss is in Estonia 10.3% (12.6 MWh/a), in Berlin, Germany 10.2% (8.2 MWh/a) and in Rovaniemi, Finland 8.0% (16.5 MWh/a) (Fig. 8).  As the temperature setpoints for ti are ts are increased, the extra room heating energy percentages decrease and absolute values increase similarly in each climate region. Without DTC extra loss, using more common ti +23°C for indoor air temperature, total room heating energy increases 57.2% in Estonia. It increases the most in Berlin, Germany by 80.7% and 37.8% in Rovaniemi, Finland as being more sensitive in warmer climate and less sensitive in colder climate region. In the context of Estonia for the smaller (45 m²) apartment the DTC extra loss results express 6.78 € (including partial general area 1.12 €) and for the larger (70 m²) apartment 10.55 € (including partial general area 1.74 €) annual additional billings. The extra billings for higher ti +23°C instead of +21°C room heating energy is approximately 6 times higher than the DTC extra loss per m². The simulation results based on long-term weather data from Tartu, Tõravere show total room heating energy decrease tendency through decades (Fig. 9). Simulations show extra room heating energy need due to the DTC thermal bridge from 1.51 to 2.51 kWh/(m²×a) with reference building base model. The range for higher indoor air temperature ti +23°C is from 1.95 to 3.04 kWh/(m²×a) and for both higher indoor air temperature ti +23°C and ti +19°C is from 1.83 to 2.89 kWh/(m²×a). The third comparison included a sample of parameter changes for the component sensitivity analysis on EPV. Lowering installed lighting wattage resulted in 2.4% decrease on EPV. Switching to lower window thermal transmittance value (0.9%), determining a lower envelope air permeability (0.8%) and switching to radiators (0.6%) also helped to gain a better EPV.  Apartment-based rotary heat exchanger (0.6%) and plate exchanger (7.2%) equipped air handling units as well as using electrical ventilation heater (13.7%) for central air handling unit, achieved a higher EPV. Replacing the domestic hot water system with electrical heating, the increase (29.7%) is the highest from the third comparison, being smaller than second comparison worst options as solid fuel or peat boilers (31.1%). Changes to apartment-based rotary heat exchanger equipped air handling unit, switching to radiators, lowering window thermal transmittance and air leakage values maintain the similar percentage to the EPV as DTC extra loss.
The second and third comparison show illustratively, how small parameter changes in the calculations of building energy performance can change rapidly the overall results of EPV. Combining DTC extra loss with other options from second or third comparison merge the outcome of decrease or increase of the EPV. However, reaching high expectancy goals on building energy performance for low energy and nearly zero energy building (nZEB) requirements, all building envelope and MEP parameters must be taken into account. Fig. 11 also indicates, that reaching nZEB requirement (105 kWh/(m²×a)) without producing energy on-site (e.g. with photovoltaic panels) is rather difficult. According to the prerequisite set for reaching nZEB EPV, only 2 options from the second comparison and 5 options from third comparison of the EPV analysis maintain the assumption for meeting minimum requirements for new buildings in Estonia. It is a substantial, that the reference building base model in this study meets the low energy building criteria, with rather exemplary envelope and MEP system parameters available. This includes the PCSP with 200 mm layer of PIR insulation material with a low thermal transmittance value of 0.11 W/(m²×K). Therefore, ensuring low thermal transmittance value, PCSP's diagonal tie connector joints must be properly insulated to be consistent with calculated thermal transmittances or additional room heating energy losses must be taken into account in energy calculations.
This study was based only on one sample building analysis. Therefore, more case studies need to be investigated for more solid conclusions regarding building extra room heating energy due to the DTC thermal bridge with operational cost analysis. In addition, simplification for insulation material as PIR was made, as it is forbidden to use it in high-rise buildings in Estonia due to the fire safety regulation [19]. Further research should also include manufacturing extra cost analysis for ensuring high quality DTC insulation methods during PCSP production.

Conclusions
The aim of this paper is to fill the gap of the energy performance and energy operational cost impact for PCSP's diagonal tie connector Ψ analysis results. In Estonian climate the thermal bridge caused an increase of total room heating energy of 10.3% (2.14 kWh/(m²×a)). The effect on energy performance value of the studied building was lower at 1.39 kWh/(m²×a), which accounted for 1.1% of the whole EPV. The influence on the annual room heating costs was relatively minor at 6.78 €/a increase per 45 m² apartment and 10.55 €/a per 70 m² apartment. The additional room heating energy and cost effect increased, when indoor air temperature setpoint is risen form standardized +21°C to more common +23°C. Analysis using 48-year Estonian climate data showed that the effect is decreasing over time. Regional climate impact analysis results exhibited larger effect in colder climate in Rovaniemi, Finland and smaller effect in warmer climate in Berlin, Germany. However, the share of extra losses was relatively similar: in the range of 7.0-10.3% for all studied regions.
In conclusion, we consider that PCSP's diagonal tie connector joints must be either: a) properly insulated to be consistent with calculated thermal transmittances, or b) additional losses must be taken into account in energy calculations.