Sensitivity analysis

Sensitivity analysis is a useful process for understanding how each value (whether known with certainty or an assumption) affects the overall result. Internal demand temperature has been identified as the most significant parameter in sensitivity analyses published in other papers (Blight & Coley 2013; Firth et al. 2010; Hughes et al. 2013), followed by heating system efficiency, external temperature, total floor area, storey height and daily heating hours (Hughes et al. 2013). A local sensitivity analysis is conducted on the model used in the present work using a one-at-a-time approach as adopted by Firth et al. (2010). The process for this sensitivity analysis is to vary key input parameters by a small increment above and below the initial set value for the parameter. The model is run for each of these new values and the change in the output parameter is calculated. From these values, a sensitivity coefficient is calculated according to equation (5-1) which is normalised to make these values comparable regardless of the scale and units used according to equation (5-2).

𝜕𝑦𝑖 𝜕𝑘_𝑗≈ 𝑦_𝑖(𝑘_𝑗+ ∆𝑘_𝑗) − 𝑦_𝑖(𝑘_𝑗− ∆𝑘_𝑗) 2∆𝑘_𝑗 𝑖 = 1, … , 𝑛 𝑎𝑛𝑑 𝑗 = 1, … , 𝑚 (5-1) 𝑆_𝑖𝑗 =𝑘𝑗 𝑦_𝑖 𝜕𝑦𝑖 𝜕𝑘_𝑗 (5-2)

Where 𝑦_𝑖 is the ith _{output variable}

𝑘_𝑗is the jth _{input parameter}

𝜕𝑦𝑖

𝜕𝑘𝑗 is the sensitivity coefficient for output value 𝑦𝑖 and input parameter 𝑘𝑗

𝑦_𝑖(𝑘_𝑗± ∆𝑘_𝑗) is the value of 𝑦_𝑖 when the input parameter has increased or decreased by a small increment ∆𝑘_𝑗

The size of the incremental change is recommended in literature as 1 % of the initial value. However, a small value such as this can lead to rounding errors in the output value. The linearity of the model response will also affect the sensitivity result; if the model is linear with respect to the input variable, the size of the incremental change will be less important than if the model is non-linear with respect to the input variable. For the sensitivity analysis in the present study, two values are tested, corresponding to ∆𝑘_𝑗 = 1 % and ∆𝑘_𝑗= 5 % of input variable. The calculated values of the corresponding sensitivity coefficient will be compared to each other to determine if the inputs are linear or non-linear as shown in Table 5-18. A criterion for linearity is chosen as a difference in sensitivity coefficient of less than 3 % between an incremental change (Δ𝑘) of 1 % and 5 %. The sensitivity coefficients of each input variable are then compared to each other and this is presented in Figure 5-9. Further details of all calculations are given in Table. C-7 and Table. C-8 in Appendix C.2.

Sensitivity analysis is applied to the working family and daytime present couple occupancy patterns to compare whether the service demand affects the results.

Table 5-18 Assessment for linearity of normalised sensitivity parameters for EEMs for working couple and

daytime-present couple occupancy patterns. Key: BI: Before intervention; AI: after intervention.

Model Parameter

Normalised Sensitivity coefficient, Sij

Linearity

Working Family Daytime Present Couple

1 % 5 % Diff. 1 % 5 % Diff.

House Temperature °C 1.120 1.121 0.1 % 1.540 1.000 0.0 % L

Night time Temperature °C 0.851 0.840 -1.3 % 0.463 1.011 -1.1 % L

Ground Temperature °C -0.126 -0.126 0.0 % -0.115 1.000 0.0 % L

Next door temperature °C -0.242 -0.242 0.0 % -0.222 1.000 0.0 % L

Boiler efficiency (BI) - -1.000 -1.003 0.2 % -1.003 0.998 0.2 % L

Radiator power kW 0.000 0.000 0.0 % 0.000 0.0 % L

Heating season length days 0.576 0.523 -10.2 % 0.536 1.130 -13.0 % N

Wall U-value (BI) W/(m2_{K) 0.351 0.250 -40.3 % 0.263 1.403 -40.3 % N}

Wall U-value (AI) W/(m2_{K) 0.077 0.050 -54.8 % 0.052 1.548 -54.8 % N}

Roof U-value (AI) W/(m2_{K) 0.025 0.027 7.1 % 0.027 0.928 7.2 % N}

Floor U-value W/(m2_{K) 0.123 0.117 -5.2 % 0.124 1.055 -5.5 % N}

Infiltration rate ach 0.172 0.169 -1.3 % 0.172 1.013 -1.3 % L

Figure 5-9 Normalised sensitivity parameter comparison of EEMs for working couple and daytime-present

couple occupancy patterns

-1.50 -1.00 -0.50 0.00 0.50 1.00 1.50 2.00

House Temperature Night time Temperature Ground Temperature Next door temperature Boiler efficiency (BI) Radiator power Heating season length Wall U-value (BI) Wall U-value (AI) Roof U-value (AI) Floor U-value Infiltration rate

Normalised sensitivity coefficient

The sensitivity coefficient is found to be linear for all parameters except building envelope U-value and length of heating season. The model is shown to be most sensitive to the temperature set-point during the daytime and night time and the boiler efficiency. The boiler efficiency has approximately negative directional proportionality whereby a 1 % increase in boiler efficiency leads to a 1 % decrease in heating consumption. House temperature is greater than proportional such that a reduction of 1 % of set-point temperature leads to a reduction in heating consumption of more than 1 %. The model is less sensitive to ground temperature, next door temperature and U-value of the building envelope and therefore the values of these parameters appear to have less effect on the results.

In comparison between the sensitivity coefficients for the working family and daytime- present couples, most parameters show little difference. The exceptions are the temperature related parameters with much greater variation. The daytime-present couple occupancy pattern shows a greater sensitivity to house temperature set-point during the day and this can be attributed to the fact that the house is occupied for a greater length of time during the day. In contrast, the daytime-present couple occupancy pattern is less sensitive to the night-time temperature than the working family occupancy pattern and this may be because more heating during the daytime results in less heating being required at night for the daytime-present couple.

5.6.3

Consideration of rebound

Energy savings following energy efficiency improvements can be compromised by rebound effects such as comfort taking. The impact of a comfort taking rebound effect on the energy savings calculated in this chapter have been investigated in the form of a comfort taking of 1 °C internal temperature increase between the initial case (at initial temperature) and following the adoption of an EEM (revised temperatures are given in appendix Table. C-9). The impact of this rebound effect on calculated energy savings is shown in Table 5-19. A loss in savings between 2 % and 6 % is calculated and this varies between the EEMs and occupancy types. Despite this loss in energy savings, a 1 °C comfort taking rebound effect does not change the ranking of any EEMs and therefore the results of this chapter. Inclusion of a rebound effect could however enable the model results to more closely represent the statistical validation results in Table 5-17.

Table 5-19 Effect of a 1 °C comfort taking rebound effect on calculated savings

Energy Efficiency Measure

Working Family Working Couple Daytime-present Couple

Initial

savings Revised saving reduction Saving savings Initial Revised saving reduction Saving savings Initial Revised saving reduction Saving

B Solid wall _Insulation 19 % 12 % 4 % 19 % 13 % 4 % 20 % 12 % 6 % C Loft Insulation 11 % 5 % 6 % 11 % 6 % 6 % 12 % 4 % 8 %

D Use of TRVs 6 % 2 % 4 % 5 % 1 % 4 % 9 % 3 % 6 %

E Use of AZHCs 11 % 7 % 4 % 15 % 13 % 2 % 11 % 5 % 6 % G Partial under-_heating - - - 18 % 15 % 3 % 17 % 12 % 5 %

Discussion

5.7.1

Comparison of energy efficiency measures

In this chapter, seven energy efficiency measures (EEMs) and combinations thereof have been compared based on how they can reduce energy consumption for a typical house over a year’s period. These EEMs fit within four approaches to delivering the energy service of heated thermal comfort with less energy demand; improved passive system, higher efficiency conversion device, improved service control and decreased service level. The service demanded has been defined according to three typical occupancy patterns; a working family, a working couple and a daytime-present couple. In single measures or in combination, improved passive system in the form of insulation showed the greatest saving potential for all occupancy patterns. This was followed by a high efficiency boiler. Energy savings achievable through heating control and service level reduction were found to vary more in relation to the occupancy type. Zonal heating controls gave greatest saving potential to the working couple occupancy type whose variable daily pattern is able to make full use of the advanced zonal heating controls (AZHC) functionality of controlling heating remotely. Partial under-heating was calculated as enabling significant energy savings for those households with only two occupants, but was not applicable to the working family who make full use of all space within the house.

These model findings provide insight for the suitability of EEMs for different householder characterisations. For householders motivated by financial savings (cost), all EEMs would be suitable as they delivered energy savings, with the exception of solid wall insulation due

to the high upfront cost. Equivalently, all EEMs are recommended as suitable for householders motivated by environmental protection (climate).

The accuracy of the present model has been assessed through comparison of modelled results with validation data from literature in section 5.6.1, and this has given insight into the reliability of this chapter’s findings. The calculated savings values for insulation measures and boiler installation were within the upper and lower quartile range of the empirical statistical data, but were 5 – 9 % higher than the median values and therefore greater validation work is required before the models could be used to confidently make recommendations on the potential for energy savings from these technologies. The discrepancy could be due to rebound effects, calculated in section 5.6.3 to cause a 4 – 8 % reduction in energy savings, or due to greater factors in heating practices not being included in the modelling (as discussed below). Energy saving calculations for EEMs of heating control and a 1 °C reduction in set-point temperature showed better agreement with validation values. A wide discrepancy was found for the level of energy savings expected for partial heating between the present model and results from another model study (Palmer et al. 2012), and therefore empirical validation is required in order to gain a better understanding of the true level of energy savings which can be expected from such a reduction in service level demand. The results of a sensitivity analysis using the one-at-a- time global sensitivity method showed that the model was most sensitive to the values of set-point temperature and boiler efficiency, agreeing with findings of other similar studies (Blight & Coley 2013; Firth et al. 2010; Hughes et al. 2013).

The variations in energy consumption predicted by the model are not as large as have been measured between similar houses (DECC 2013a; Beizaee et al. 2015), and this could be explained by the observation that occupancy characteristics include more than the occupancy patterns discussed here. Other factors, such as opening windows and doors for ventilation, and use of secondary heating will also contribute to the variations in energy consumption for different occupants (as discussed in sections 2.2.1 and 2.2.3.1). The initial state of a building is another key factor in the size of, and variation in, calculated energy savings. In this study, for the sake of simplicity, the initial state of the house and its occupants was based on a 'typical' UK dwelling, but with a larger variation in initial state, a wider range of saving would expected. The savings calculated will also depend on the extent to which EEMs are implemented.

5.7.2

Suitability of energy efficiency measures to occupants

In reality, the suitability of some EEMs will vary according to the household occupants, and this must always be taken into account when making recommendations for the adoption of such measures. The suitability of EEMs could be further informed by

consideration of heating practices as the way householders attain thermal comfort for themselves. Of the fourteen heating behaviours identified from literature in Table 2-5, length of occupied period or heating period has been most thoroughly covered in the modelling of this chapter, represented in the three occupancy patterns of working family, working couple and daytime-present couple. Temperature set-point and portion of house heated have been investigated as EEMs of lower service level; personal heating is also related to this as a lower temperature set-point can be comfortable with the addition of clothing or other adaptive measures. For households who exhibit practices of personal heating, EEMs of service level could be suitable, unless their level of service is already low. Temperature reduction may not, however, be suitable for those occupants who are elderly or suffering from health conditions, and the use of advanced heating controls will not suit all types of people (as discussed in section 2.3.4.5).

The different types of barriers posed by each EEM were indicated in Table 5-12. When applying this analysis to individual households and buildings, these further context-specific details could take the application beyond the three occupancy patterns explored in this paper, by categorising households according to age, physical ability, tenure type (owned vs rented) or capacity to overcome different barriers. Passive system and conversion device measures resulted in similar savings for all three occupancy types and therefore exhibited greatest resilience to changing households. These measures can therefore be particularly recommended in houses with a high turnover, such as the rental sector. Cost, as a major barrier to adoption of EEMs, has been addressed within this chapter by comparing combinations of low cost measures to higher cost individual measures. Cost constraints of larger measures such as passive system and conversion device upgrade will limit some households more than others. Lower cost measures tend to require greater expertise or present a greater inconvenience, and therefore will only result in maximum savings if suitably matched to the household.

5.7.3

Inclusion of additional heating practices

The inclusion of other heating practices, as identified in Table 2-5, could provide further insight into suitability of EEMs for householders with different types of behaviours. Approaches to how these practices could be included within building modelling are explained below:

 Interaction with heating controls: Type of thermostat and interaction with the thermostat have been investigated as control EEMs, and the competence of a household to use such controls will affect the adoption potential of such measures. However, thus far controls have been modelled according to their designed

operation mode, but empirical studies have revealed heating control technologies being used in adapted ways and contrary to their desired function. An example is that some occupants control heating by overriding a thermostat to use it like an on- off switch (turning up to 30 °C for ‘on’ and down to 10 °C for ‘off’). Representation of this heating behaviour could be achieved by assigning a wide temperature dead- band to the thermostat during occupied hours and could be more representative of real heating practices.

 Use of secondary heating: The use of secondary heating can be split into two types; an additional heating source (such as an electric heater) used when the primary heating source is insufficient to warm the room or house to a comfortable temperature and switched on when the room temperature is deemed too cold, or a ‘rustic’ type of heat source such as a wood or gas fire which is lit in a living space for heating and ‘aesthetic’ reasons. Secondary heating can be modelled as a heat gain, and commonly the heat input can be differentiated into convective and radiative heat; secondary heating sources typically emit more radiative power than a conventional central heating system. An aesthetic type of heating source could be modelled on a schedule, such that it is lit for a few hours every evening in the living room. For the supplementary heating type of secondary heating source, a secondary thermostat could be modelled, representing the internal thermostat of an occupant; the heat source could be controlled based on the temperature dropping below a certain level in an occupied space in which it is commonly used. The availability of conditional control within building models (whereby an action is taken depending on a statement being evaluated as TRUE or FALSE), or set-point temperature bounds as upper and lower limits, could enable more realistic representation of the conscious or unconscious decisions upon which heating practices depend. Apart from modelling, use of secondary heating as a heating behaviour could be an indication of a household’s priority for comfort over cost or environment. For a householder who is motivated by comfort, savings offered by an EEM may be negated if a sufficient level of warmth is not being delivered and heating is supplemented by the use of secondary heating; this could lead to significantly higher CO2 emissions, depending on the fuel source of the secondary heating.

 Internal door opening: The tendency of a household to leave internal doors open can be expected to impact particularly on EEMs in which different parts of the house are heated differently. Advanced zonal heating control can be compromised if internal doors are not kept predominantly shut between heated and non-heated rooms. Partial heating would also not be effective unless un-heated spaces are shut

off from heated spaces. The effect of open internal doors can be modelled by specifying internal air flow between air nodes or zones in the model. However, it may be more effective to simply avoid recommending these zonal EEMs for occupants who have a preference for keeping internal doors open.

 Window opening: Open windows can be modelled as a natural ventilation by specifying a ventilation rate of air at the external temperature. Specifying the rate of ventilation is non-trivial as it relies on factors such as internal and external temperatures and external wind speed. Jack et al. (2015) found a linear relationship between window open area and ventilation rate, with an increase of 2 ach (for the whole house) between window closed and window open by 0.6 m2_{, extrapolated to} 3.8 ach per m2 _{of opening area. Infiltration is expected to be higher at higher} external wind speed (compared to average wind speed of 2.0 m/s on the day of the test), however window opening behaviour was found to be less likely at higher external wind speeds (Fabi et al. 2012).

With the inclusion of such heating practices, the accuracy of the savings prediction may be enhanced, or alternatively, the range of expected savings may be widened. This wider expected range could be displayed graphically in the results figure. Figure 5-10 shows such an example, for the case when the occupancy pattern of the household is uncertain and therefore all three occupancy patterns of working family, working couple and daytime- present couple are displayed on the same plot. The inclusion of a greater number of heating practices would make the range wider and better represent the variety of savings which could be expected following the adoption of an EEM.

Figure 5-10 Range of modelled energy savings for single and combinations of energy efficiency measures

representing the uncertainty of energy savings if occupancy pattern is unknown. EEMs letters are explained in the Key in Table 5-15

Conclusion

The aim of this chapter has been to investigate the effectiveness of energy efficiency measures (EEMs) in reducing the energy required to deliver the service of heating thermal comfort within a typical UK house. As in Chapter 4, EEMs were compared based on the energy demand calculated to deliver a given functional unit of service, in this case a year of thermal comfort as defined by the occupancy pattern, and the energy savings which each EEM could deliver.

Although savings with the full passive system improvements (wall and roof) were calculated to be the highest for each occupancy pattern, they were also identified as being