Phd candidate Harold E. Huerto C.
XXXII Cycle
Politecnico di Milano
Department of architecture, built environment and construction engineering
Milestone 3 – Position report
April 2018
Assessment of energy and environmental performance of historic building: tools and methodology
Tutor: Claudio del Pero
Supervisor: Fabrizio Leonforte
Summary
Historical buildings are a part of a territory’s cultural heritage, they represent its identity, history, and are an important resource.
Compared to today’s buildings, some of them are of considerable dimensions, of singular architecture complexity and are realized with obsolete construction techniques.
For these reasons historical buildings are often subject to peculiar physics phenomena that make them difficult to compare with common buildings making their study particularly difficult.
Usually these structures contain a lot of objects and works of great artistic and cultural value made with several materials, for example: artifacts, artworks, sculptures, frescoes, organs etc. The presence of these objects of such a different nature makes the preservation of the entire organism even more complicated.
In particular, the study of the environmental behavior of these buildings is a task that requires a deep analysis of the environmental properties (as temperature, humidity ratio, wind speed, etc.) throughout the seasons, to define a monitoring strategy using non-invasive tools.
In support of this kind of studies, the researchers could realize virtual models that can be used to evaluate possible active or passive strategies and solutions aimed at ensuring the best preservation of the structure and its objects, as well as to improve the comfort of people that visit such spaces.
These simulation models require a phase called calibration that allows to reduce the initial discrepancy between the measured and simulated data. However, this process is generally used in buildings with systems, comparing the energy consumption obtained from the bills. In the historical buildings without HVAC systems, other properties have to be considered to perform the calibration. Once the model has been calibrated, it is possible to reproduce the microclimatic behavior of the building with a good approximation and to evaluate the effect of different scenarios for the improvement of the actual environmental and energy conditions.
Keywords: Historic building; Microclimate; Monitoring methodology; Building simulations; Calibration process; Uncertainty analysis; Sensitivity analysis; Residual analysis; Model validation; Risk analysis.
The state of the art regarding the assessment of energy and environmental performance in historical buildings, can be carried out through the monitoring of the environmental indoor properties and the realization of a virtual model calibrated comparing the data acquired previously, whose purpose is to predict the effects of possible solutions oriented to improve the actual condition. These phases are complementary to each other.
Environmental monitoring
The hygrothermal behavior of historical churches is often the result of complex energy balances due to many different causes. The environmental characterization of these buildings represents a task that requires a deepen and carefully planned study based on the analysis of specific problems to be solved. In detail, the European reference standards concerning the measurement of environmental parameters of cultural heritage [1][2], do not define a precise methodology to be used, but describe general procedures based on the phenomenon that one wants to investigate.
In this sense, over the years, the surveys of historic buildings have been carried out by applying different methodologies and tools. The following paragraph describes the main procedures adopted for the monitoring and the microclimatic characterization of some European historical churches.
In the monitoring carried out by M.J. Varas-Muriel et al. at the Church of San Juan Bautista, in Spain [3], the effect on the microclimate of the traditional heating system was investigated. For this purpose, nine balloons were inflated with helium gas and distributed at different points in the church. To each balloon 6 to 7 temperature and relative humidity sensors have been fixed with an inter-distance of 1,5 meters from each other. The data was surveyed during a day in February and one in March, in a time interval of about 4 hours, with a sampling time of 1 minute. The air flow from the system was measured by an air speed meter.
In the microclimatic analysis of the church of St. Christopher in Lisbon [4][5], numerous sensors were used that allowed to measure the internal distribution in plan and in elevation of temperature and relative humidity of the air. The sensors used in this study respect the minimum requirements suggested by the EN 15758 and EN 16242 standards on the instrumentation to be used for the environmental monitoring of cultural heritage.
At Santa Maria Nascente in Agordo in the Italian Alps, the monitoring carried out by Camuffo et al. [6] illustrates some detection methods for the microclimatic characterization of historic churches. In particular, a distinction is made between long-term monitoring through sensors equipped with data loggers and punctual monitoring (spot) in which the data are manually taken during a specific period in order to verify particular conditions. Specifically, the measurement of the profiles of temperature and relative humidity of the raised air was carried out through sensors fixed to the ropes and without the enclosures in order to improve the response time. The relief of surface temperatures was made using IR thermometers and quasi-contact thermometers for the most accurate measurements. Finally, some thermal images of some surfaces were made through an infrared camera to evaluate the impact of hot air on the surfaces of objects made of different materials.
In the study carried out in the church of San Francisco de Asís in Seville in Spain [7], in order to obtain environmental data representative of the internal microclimate, some sensors were positioned in areas not influenced by sources of radiation, air flows coming from external, etc. The external environmental parameters were instead acquired by the weather stations available closest to the building.
The various procedures described above underline the need to install a large number of sensors, spatially distributed in a uniform manner, often hooked to ropes suspended by balloons or fixed to the roof of the building. This system, although it allows to represent the distribution of environmental properties in a uniform and detailed manner, involves the physical and visual occupation of some areas or of the whole building for a specific period.
The collect of the environmental data allows to characterize the indoor microclimate of the building and through the comparison with the results of the simulation allow to perform the model calibration.
Simulation models and calibration process
Nowadays, there are many software that allow architects, engineers and researchers to realize simulation models, which allow to reproduce the thermophysical behavior of the analyzed building. Some of the most common and validated software are Energy Plus and Trnsys. Realizing this kind of model requires a good knowledge of the building, about its main physical characteristics and the operating conditions, which often makes the calibration a difficult and time consuming process.
There are many problems related the modelling of the building in energy simulations and they increase when we are dealing with historical buildings. In most cases there isn’t specific information about the material properties, the ventilation flow rate, internal gains due to people etc., furthermore generally these information is not easy to collect. Compared with new buildings, the architectural complexity that usually characterized the historical ones, constrain to do some simplifications during the modelling phase, with an increase of the uncertainty.
The calibration of energy models is a highly developed theme on buildings equipped with HVAC system. The purpose of these models is to evaluate different energy retrofit interventions, to support during the life of the building, as well as to facilitate the decision-making process with respect to possible maintenance or improvement interventions.
The main references regarding the calibration process are the following:
- American society of heating, refrigerating and air conditioning engineers (ASHRAE).
- International Performance Measurement and Verification Protocol (IPMVP).
- M&V guidelines for FEMP.
These documents mainly describe the methodology to be used to calibrate the energy models of buildings, based on their monthly consumption, through the analysis of the bills. For constructions without HVAC systems, as are many historical structures, the air temperature during the calibration process is used, however, this aspect is not particularly detailed and there are still no guidelines that orientate towards the correct calibration of such buildings.
The implementation of energy models requires a lot of input data and some assumptions must be made in the absence of information. Where possible, investigations may be carried out to obtain some useful information on the building, which can be used in the calibration. Different calibration levels can be defined based on the available information. All levels are based regardless of the analysis of the bill that must be made for at least a year, however the calibration will be more accurate if more information is available.
The following list represents the 5 calibration levels:
- level 1: based on incomplete information but as built data;
- level 2: by visiting can verify as built data and obtain more information;
- level 3: based on detailed audit of case study;
- level 4: based on short term monitoring;
- level 5: based on long term monitoring.
Several issues can affect calibration procedure that could be summarize in the following points, according to [8]:
- Standards: lack of explicit standards for calibration criteria – current guidelines only specify acceptable error ranges for yearly whole-building simulation, but do not account for input uncertainty, sub-metering calibration, or zone-level environmental discrepancies.
- Expense: the expense and time needed to obtain the required hourly sub-metered data, which is usually not available.
- Simplification: calibration is an over-specified and under-determined problem. There are thousands of model inputs but relatively few measurable outputs with which to assess the model accuracy.
- Inputs: lacks of high-quality input data required for detailed models
- Uncertainty: there are currently few studies which account for uncertainty in models input and predictions, thus leading to a lack of confidence in building energy simulations outputs.
- Identification: problems identifying the underlying causes of discrepancies been model predications and measured data.
- Automation: lack of integrated tools and automated methods that could assist calibration.
Some statistical indices allow to determine the goodness-of-fit of the building energy model in order to establish the accuracy of the calibration. After calibration has been endorsed as methodology for building simulation, statistical indices have become international references for validation of calibration.
Commonly the Mean Bias Error (MBE) and the Coefficient of variation of the Root Mean Square Error (Cv(RMSE)) are the two statistical indices used. The consideration of both indices allows preventing any calibration error due to errors compensation.
MBE (Mean Bias Error) is one of common index which can define whether the model is calibrated or not. MBE measures how closely simulated data corresponds to monitored data. MBE is calculated by following formula.