Study of Residential Thermal Transfer Values (RTTV) and vertical daylight factors (VDF) for Heavily Obstructed Public Housing Flats

. Hong Kong, one of the most densely populated cities in the world, consumes more than a quarter of its total energy in residential buildings. The high urban density leaves many residential flats in a heavily obstructed environment ， seriously affecting indoor lighting performance. In response, the Hong Kong government issued requirements for residential thermal transfer values (RTTV) and vertical daylight factors (VDF). The control of RTTV and VDF is considered to be effective in improving the energy efficiency and daylighting performance of residential buildings. However, achieving the required RTTV and VDF at the same time can be conflicting in terms of building façade design. This paper studies the daylight performance and energy use of public housing flats facing large sky obstructions through computer simulations. General correlations between RTTV, energy consumption, and daylighting performance are investigated.


Introduction
Hong Kong is one of the world's most populated cities, so meeting its housing needs can be difficult. The growing number of people and the limited amount of land have made the majority of residential buildings high-rise flat blocks in dense urban areas, blocking a lot of the sky [6]. In addition, the majority of residential buildings have been equipped with air conditioning units due to the rise in living standards and the demand for comfort in the past. This resulted in a strong demand for controlling residential buildings' thermal performance.
RTTV was implemented in 2014 [3]. RTTV makes the assumption that air conditioning operates from April to October and does not specifically take into account the heating season [3,6]. For residential buildings, the RTTV should not exceed 14 W/m2 for walls (RTTVwall) or 4 W/m2 for roofs (RTTVroof) in order to increase their energy efficiency. Similar to OTTV [9], the design of the building envelope does not include daylighting credits, although the Hong Kong Government requires RTTV calculations to be carried out when building plans are submitted for approval.
In order to obtain approval, the Buildings Department established a performance-based strategy in 2003 [1]. This method specifies the minimum VDF requirements for kitchens and habitable rooms, which must not be less than 4% and 8%, respectively.
The Library of Modular Flat Design (MFD) has been adopted by the Hong Kong Housing Authority since October 2008 for all domestic public housing blocks specifically designed to maximize development potentials and strike a better balance among several factors, such as the optimal use of valuable land resources, building efficiency, cost-effectiveness, and the needs of occupants. Additionally, the design library has a lot of potential for flexible façade design for buildings.
The general correlation between RTTV and daylighting performance and the energy consumption is examined in this paper using computer simulation techniques to investigate the daylight performance and energy consumption of a severely obstructed residential flat.

Simulation Approach
This study applied Ladybug Tools to simulate the daylighting performance and energy consumption of the residential flats separately by invoking Radiance [7] and EnergyPlus [4] in the platform of Rhinoceros & Grasshopper [10]. As simulation engines, Radiance and Energyplus offer powerful functions to simulate the effects of various design features on daylighting performance and energy consumption and are the recommended simulation programs for the Hong Kong Government [5]. Ladybug Tools is an open-source software that reads and writes geometry, material, and construction codes and exchanges data between platforms and simulation engines [10].

Building Description
In highly crowded Hong Kong, most developments are packed with high-rise buildings built close to each other with little room to maneuver, resulting in unavoidable obstruction [8]. A 36-story residential building utilized for the simulation has 18 flats per floor. The lowest VDF value for the habitat room of 9.71% is found on the ground floor of Flat 3, which conducts further study. The residence flat faces north, and a 36-story building is 25 meters opposite. The floor plan and context for Flat 3 as shown in Figure 1. Buildings in Hong Kong are required to comply with assessment tools such as the Building Energy Code (BEC) [6] and BeamPlus [2]. Tables 1 and 2 show the building and facade information used for the simulation.

Weather file
The EnergyPlus conducts annual hourly calculations using measured weather data. A weather database is necessary for a simulation model in order to obtain and compare simulation results for different scenarios. This study uses the weather file CityUHK-45007, which is recommended for RTTV [3].

Results
The simulation model included nearby buildings and external shading elements to simulate shading effects. In order to get some ideas about the relationship between RTTV and building performance, additional fenestration designs are included in the simulations, shown as Table 3. The average daylight factor (ADF) of living and dining room on 1/F and 36/F are simulated. In order to better understand the relationship between RTTV and light performance, the average daylight factor was correlated with RTTV. Figure 2-3 shows that the daylight factor is almost linearly related to the RTTV. A slight non-linearity occurs at 1/F, which may be due to changes in the external reflectance component caused by lateral window opening variations.
Similarly，the simulation runs with energy schemes conducted, and the simulated results were used for the regression analysis. Figures 4-5 show a negative correlation between lighting energy consumption and RTTV, and a positive correlation between air conditioning energy consumption and RTTV. This shows that for a given glazing type, the key variable that determines the solar heat gain and the amount of daylight entering the building is the glazing area. A large window area leads to a greater cooling demand due to increased solar gain， while introduces more natural light into the indoors and reduces lighting electronic consumption.
The R² are ranging from 0.96 to 0.99, which indicates that more than 96% of the changes in ADF and energy consumption can be explained by the variations in RTTV.

Daylight Performance
To better understand the interaction between the external light environment and the indoor lighting performance, VDF, ADF, and Percentage of hour indoor daylight illuminance exceed design indoor illuminance (IED) during daytime for each of the five floors(i.e. 1st, 6th, 11th, 21st, 26th, 31st and 36th) of the living and dining room were simulated and the intra-group correlation was analyzed using, see Figure 6-8. The R ² are ranging from 0.958 to 0.998, which indicates that the three index have strong correlations with each other.

Energy
Taking advantage of the MFD, the façade of Flat 3 was optimized based on total energy consumption, as shown in Table 4. Figure 9 shows that the optimized façade design (OPT) reduces the air conditioning energy consumption by 1% and the lighting energy consumption by 20% compared to the reference case (RC).

Conclusion
This study researched general correlations among RTTV, daylighting execution and energy consumption. A newly constructed residential flat with heavy obstruction has been carried out to simulations. The government's daylight and thermal requirements are met by the design of the façade. RTTV has a strong correlation with daylighting performance and energy consumption for a given building façade type. This is a demonstration study of RTTV; additional researches are required to investigate the connection between RTTV, visual comfort, and energy consumption in buildings in order to assist building designers in early design optimization.