Building Services Engineering Research & Technology

Heat exchange between chilled food storage and conditioned spaces in large food retail stores is not currently required as part of design stage regulatory compliance energy performance models. Existing work has identified that this exchange has a significant impact on store energy demand and subsequently leads to unrealistic assessment of building performance. Research presented in this article uses whole building dynamic thermal simulation models that are calibrated against real store performance data, quantifying the impact of the refrigeration driven heat exchange. Proxy refrigerated units are used to simulate the impact of these units for the sales floor areas. A methodology is presented that allows these models to be simplified with the aim of calculating a realistic process heat exchange for refrigeration and including this in thermal simulation models; a protocol for the measurement of chilled sales areas and their inclusion in the building models is also proposed. It is intended that this modelling approach and the calculated process heat exchange inputs can be used to improve the dynamic thermal simulation of large food retail stores, reduce gaps between predicted and actual performance and provide more representative inputs for design stage and regulatory compliance energy calculations.

Practical application: The modelling methodology and research findings presented in the article are of practical use for building energy modelling engineers using dynamic simulation models to design and/or evaluate the energy performance of large food retail stores. The methodology can be used in the design of new facilities or the evaluation of large scale retrofit projects. It is also of practical interest to energy and facility managers within large food retail organisations as it will aid their understanding of applied energy performance models.

In the UK, approximately 16% of the energy use can be attributed to domestic wet central heating systems. Government financial support and advances in technology have led to boilers becoming more efficient and a range of technologies are now available that claim to be able to improve the efficiency of domestic wet central heating systems. One such low cost technology is a passive deaerator. This article presents the results obtained from installing a passive deaerator on the closed loop of a gas-fired wet central heating system, under controlled conditions in the Salford Energy House. The results indicate that although marginally less heat output was required from the boiler when the passive deaerator was operating, these savings are more or less out weighted by the boiler short cycling more frequently. Consequently, the overall reduction is gas consumption achieved by utilising the passive deaerator device is only of the order of 0.5%; this scale of savings may just be a consequence of measurement noise. The implications are that although a marginal benefit may be attributed to these products, if short cycling takes place, then these savings may become insignificant.

Practical application: This article describes a test method that has been used to quantify the energy savings that could be achieved by installing a passive deaerator on the closed loop of a wet central heating system. Although the results indicate that the energy savings associated with using such a device are likely to be marginal, the test method described could be used to test a range of other devices that claim to improve the performance of domestic wet central heating systems, to directly compare before and after performance.

In this paper, a hypothetical district representing the typical urban districts in Hong Kong was considered and a district cooling system model was designed for this district. Mathematical models were tailor-designed for all the major district cooling system equipment to simulate the effects of changing the pumping station’s configuration on the energy performance of the district cooling system. The measures included the use of multiple pumping stations and an unequal number of pumps in each station. In view of the vast number of pumping station combinations possible for analysis, a hydraulic gradient evaluation method was adopted to assist a quick assessment and exploration of those combinations that would be technically feasible. Furthermore, the energy performance of all these technically feasible combinations was evaluated to identify an optimum design that would lead to the lowest electricity consumption.

Practical application: In a district cooling system where there is only one main pumping station for distributing chilled water to all the buildings in the district, the chilled water flow rate and pressure head are very high. Adding booster pumping stations can help to reduce pressure head, pump size and hence power demand of the main pumping station. In this paper, the effects of different pumping station configurations on the energy performance of a district cooling system were investigated. The configuration that could mitigate the impacts of a low delta-T on the energy performance of the district cooling system was also identified.

This study experimentally investigated the operating characteristics of the variable frequency speed-regulating system of a water pump in a closed system. The test data for the variable frequency water pump under various conditions were obtained by changing the water pump frequency and the opening setting of a regulator valve in the system. This study revealed that the total power ratio of the water pump system before and after the frequency adjustment did not exhibit cubic correlations with its rotational speed ratio as defined by the affinity law, especially in areas other than the high-efficiency area of pump operation. The equations that express the correlations between the frequency ratio and the total power ratio under the full range of operating conditions were obtained in this study by using the data fitting method. The test confirmed that the fitted characteristic equations had a higher precision and a wider scope of applicability than the existing equations when the water pump, motor, and variable frequency drive were considered as a whole.

Practical application: The variable frequency speed-regulating system of a water pump was used in cooling, cooled and heating systems. However, the water pump power was calculated by the standard cube law, and its energy-saving potential was exaggerated. This study revealed a new energy model of a water pump system that can more accurately calculate the total power of the variable frequency water pump in practical engineering.

Monthly utilisation factors are the basis of many procedures for calculation of monthly heating or cooling requirements for buildings, notably in the procedure described in standard ISO 13790:2008 ‘Energy performance of buildings – Calculation of energy use for space heating and cooling’, which is widely used for the implementation of the Energy Performance of Buildings Directive in Europe. The procedures used to determine the values of the factors are invariably empirical rather than being derived from first principles, with the principal parameter being the ratio between monthly mean heat gains and monthly mean heat losses for the space in question. This article shows that this ratio is inherently insufficient to define the values and illustrates how months with similar values of the ratio can have different utilisation factors. It also shows that, if daily heating requirement is proportional to outdoor temperature, the key building parameter needed to determine the utilisation factor is the familiar base temperature. The base temperature can be expressed in terms of the monthly gain: loss ratio and the mean indoor and external temperatures: the day-to-day frequency distributions of outdoor temperature is also important. Finally, the article demonstrates that, for many situations, the ISO 13790 procedure and a linear model with residuals produce similar estimates of monthly heating requirement. However, this is not true towards the upper end of its observed range. In this situation, the linear model produces lower values for utilisation factors and correspondingly higher heating (and cooling) requirements. This effect is most marked when the mean indoor and outdoor temperatures are close or the space is well-insulated (causing a given heat gain to represent a higher potential temperature difference).

Practical application: Monthly utilisation factors are the basis of many procedures for the calculation of monthly heating or cooling requirements for buildings, notably in the procedure described in standard ISO 13790:2008 ‘Energy performance of buildings – Calculation of energy use for space heating and cooling’, which is widely used for the implementation of the Energy Performance of Buildings Directive in Europe. This article shows that an alternative approach based on the concept of energy signatures, although producing very similar results in many situations, is a more robust and extendable basis for monthly heating and cooling energy demand calculations.

Saudi Arabia’s energy consumption is increasing astronomically. Saudi Building Code prescribes a fixed base temperature of 18.3℃ to estimate the heating degree-days and cooling degree-days. Using historical meteorological data (2005–2014), this article presents the heating degree-days and cooling degree-days estimated for the representative cities in all the five inhabited climatic zones of Saudi Arabia. We used the base temperatures of 14℃, 16℃ and 18℃ for heating degree-days, and 18℃, 20℃, 22℃, 24℃ and 28℃ for cooling degree-days for Dhahran, Guriat, Jeddah, Khamis Mushait and Riyadh cities. We developed multiple regression models for heating degree-days and cooling degree-days at various base temperatures for these zones. Degree-days for other cities in similar climates with limited input data can be computed with these. Lowering of base temperature by 2 K from 18℃ reduced the heating degree-days by 33–65%. At 14℃ of base temperature, the heating requirement reduced by 60–95%. Elevating the base temperature by 2 K from 18℃ lowered the cooling degree-days by 16–38%. At 28℃ of base temperature cooling can be completely eliminated in Khamis Mushait, and reduced by 65–92% in other cities. This observation merits rethinking about use of appropriate base temperatures that properly link the outdoor environment to reduce the energy consumption.

Practical application: Using historical data, we developed regression models for predicting heating and cooling degree-days for five cities of Saudi Arabia in various climate zones without the historic data. Using these, we can estimate the changes in heating/cooling load due to the variation in base temperatures. For example, lowering base temperature by 2–4 K from 18℃ reduces the HDDs by 33–95% and elevating the base temperature by 2–4 K from 18℃ lowered the CDDs by 16–68%.

One of the most discussed issues in the design community is the performance gap. In this research, we investigate for the first time whether part of the gap might be caused by the modelling literacy of design teams. A total of 108 building modellers were asked to comment on the importance of obtaining and using accurate values for 21 common modelling input variables, from U-values to occupancy schedules when using dynamic simulation to estimate annual energy demand. The questioning was based on a real building for which high-resolution energy, occupancy and temperature data were recorded. A sensitivity analysis was then conducted using a model of the building (based on the measured data) by perturbing one parameter in each simulation. The effect of each perturbation on the annual energy consumption given by the model was found and a ranked list generated. The order of this list was then compared to that given by the modellers for the same changes in the parameters. A correlation analysis indicated little correlation between which variables were thought to be important by the modellers and which proved to be objectively important. k-means cluster analysis identified subgroups of modellers and showed that 25% of the people tested were making judgements that appeared worse than a person responding at random. Follow-up checks showed that higher level qualifications, or having many years of experience in modelling, did not improve the accuracy of people’s predictions. In addition, there was no correlation between modellers, with many ranking some parameters as important that others thought irrelevant. Using a three-part definition of literacy, it is concluded that this sample of modellers, and by implication the population of building modellers, cannot be considered modelling literate. This indicates a new cause of the performance gap. The results suggest a need and an opportunity for both industry and universities to increase their efforts with respect to building physics education, and if this is done, a part of the performance gap could be rapidly closed.

Practical application : In any commercial simulation, the modeller will have to decide which parameters must be included and which might be ignored due to lack of time and/or data, and how much any approximations might perturb the results. In this paper, the judgment of 108 modellers was compared against each other. The results show that the internal mental models of thermal modellers disagree with one another, and disagree with the results of a validated thermal model. The lessons learnt will be of great utility to modellers, and those educating the next generation of modellers.