Energy Efficiency in Homes

The ability of a building to require low energy to provide comfort is the first of the two conditions required to reach the concept of a low energy building.

Interactive investigation of building energy performance

The work presents a methodology, based on a simulation model and graphical figures, for interactive investigations of building energy performance. The investigated examples illustrate how decisions in the early stages of the building design process can have decisive importance on final building energy performance in United States’ (US) climates.

1. Introduction

The residential sector is responsible for approximately 40% of the total world energy consumption [1, 2, 3], with space heating/cooling accounting for a majority, with a share of between 61% and 70% of home energy use [4]. In this context, there is a need for looking at the potential of low energy buildings, in order to reduce residential energy use and thus its related GHG emissions [5, 6].

The ability of a building to require low energy to provide comfort is the first of the two conditions required to reach the concept of a low energy building. Indeed, buildings should also be able to generate sufficient energy from renewable sources to cover their demand [7, 8]. Amongst the renewables, solar energy has a significant advantage over the others as it is more predictable and is abundant in some regions.

For the purpose of this study, we developed a numerical modeling tool able to simulate energy use in buildings across different climate locations in the US. The results from a “base-case home” (BCH) are compared to the energy use of a hypothetical “energy-efficient home” (EEH), consisting of higher energy performance envelope components, in order to show the degree of impact of several factors affecting the energy usage. These include, among others; building envelope materials, site characteristics and local environments.

2. Methodology

The analysis structure is presented below and summarized in Figure 1.

Figure 1: The overall structure of the building energy simulation [9]

2.1. Climate data

Four climatic locations in the US were evaluated In the study (Table 1). The desired indoor air temperature is set to 20˚C in winter (T_in,w) and 23˚C in summer (T_in,s) in order to maintain an adequate level of comfort.

Table 1: Climate conditions of four US locations [10, 11]

2.2. Building geometry and orientation

A one storey house of 56 m² surface floor area was chosen. Figure 2 shows the house plan. The household is assumed to be occupied by a family of four (two adults and two children).

Figure 2: House floor plan

2.3. Characteristics of building envelope materials

For the purpose of the study, we proposed two types of home, one with typical materials (BCH), and another one with higher quality building envelope materials (EEH) (Tables 2 and 3). The framework and dimensions were identical for each home in question.

Table 2: Physical characteristics of the Base-case Building envelope

Table 3: Physical characteristics of the Energy Efficient Building envelope

Linear and punctual thermal transmittance values were assumed to represent 10% of the total building heat transmittance for both BCH and EEH.

2.4. Heating/Cooling systems

In order to estimate the energy consumption of the space heating/cooling systems, the following assumptions were made:

– The electrical radiant heaters provide uniform air temperature distribution, and their efficiency is 100%.

– The air conditioning unit has an overall efficiency of 100%, and provides uniform air temperature distribution.

3. Simulation and results

3.1. Base-case and energy efficient building comparison

Using the simulation model developed, annual space heating energy use for the two homes (E_SH,BCH and E_SH,EEH) at each location were found (Figure 3). Annual energy uses for space cooling for both building types in all regions are displayed in Figure 4.

The difference in energy use (heating and cooling) between the two homes is explained by the different envelope component U-values shown in Table 4. The impact of improved envelope U-values of the EEH in all locations is a 59% relative saving of the heating and cooling energy use (relative saving = [E_SH,BCH – E_SH,EEH] / E_SH,BCH).

Table 4: Envelope U-value comparison between Base-case and Energy Efficient buildings

Figure 3: Space heating annual energy use – Base-case and Energy Efficient buildings

Figure 4: Space cooling annual energy use – Base-case and Energy Efficient buildings

3.2. Envelope factors impacting on space heating energy use (ESH)

In order to reduce heating energy consumption, a common strategy is to add additional ceiling insulation to the building in question. The influence of insulation on energy use depends mainly on the ceiling dimensions and the insulation thickness. Figure 5 shows the ratio of the calculated energy use (E_SH) over the BCH usage (E_SH,BCH) as a function of the additional thickness of wall/ceiling insulation. The insulation thickness has a clear influence for the first 0.05 m, particularly for walls.

From an economic point of view, it is advantageous to find the optimal operating thickness of insulation. An improved U-value of the wall caused by thicker insulation does not necessarily have to be of high magnitude to have an impact on energy use. This is because of the larger area of the wall compared to other building components. For example, Figure 5 shows that the addition of 0.12 m wall insulation reduces the annual energy use for space heating by almost 10%. The same refurbishment of the ceiling reduces the annual energy use by less than 6%.

Figure 5: Influence comparison of wall/ceiling insulation thickness on heating energy use

The net effect of an opening on the heat balance of a building depends on its characteristics. Windows and doors should be able to provide effective resistance to heat flow. The heat transmittance of the openings is treated as a linear function, although in practice it varies due to the commercial availability of different types of windows and doors (see Figure 6). The replacement of windows of U-values from 5.8 (W/m²K) to 1.0 (W/m²K) reduces the space heating energy use by 24%, which is considerable, especially in regions with high heating requirements.

Figure 6: Influence comparison of window/door U-value on space heating energy use.

Another important parameter that influences the total use of energy is the air ventilation (or infiltration). An improved rate of air ventilation/infiltration from 1.7 to 0.9 in the Base-case building reduces the annual heating energy use by 19% for all locations.

4. Conclusion

This study introduced a tool that can be used for interactive evaluations of the impacts of building geometry, envelope materials and climate factors on energy use. The proposed model permits future investigations, which could utilize different parameters for energy consumption or the financial viability of small-scale renewable energy systems, depending on the specific characteristics of the city or region to be analyzed.

Despite the inclusion of climate conditions, this work emphasizes the influence of building envelope materials on energy efficiency in the home. The outcome of the introduced procedure is illustrated in graphical figures in section 3.2. These figures can be used as a support to interactively identify and validate refurbishment measures that influence energy use. In the early stages of building design, the simulation tool has the flexibility to allow testing with a series of scenarios; it can calculate efficiency for different types of envelope materials, climate conditions, and heating systems, among others.

This study proves that energy efficiency in homes is possible, by installing high performance building envelope materials, and can be an effective strategy to reach the concept of low energy buildings. If a building has the ability to require low energy to provide comfort, the size of the solar PV system able to cover its demand would be much smaller, as well as its investment cost.

By
Francois Simon, University of Granada
Prof. Aymeric Girard, University Adolfo Ibañez

References