OPTIMISATION OF AN HVAC SYSTEM FOR ENERGY SAVINGS AND THERMAL COMFORT IN A UNIVERSITY CLASSROOM Giovanni Semprini1, Cosimo Marinosci2 and Alessandro Gober3 1DIN - CIRI, University of Bologna, Bologna, Italy, giovanni.semprini@unibo.it 2 DIN - CIRI, University of Bologna, Bologna, Italy, cosimo.marinosci@unibo.it 3 CIRI, University of Bologna, Bologna, Italy, alessandro.gober@unibo.it

TOPIC OF THE STUDY: - In this work an energy dynamic simulation of two classrooms served by an HVAC system unit with a single zone distribution has been realized by means of EnergyPlus (simulation of the building-plant system) - University classrooms may have ventilation rate demand and thermal loads consistently changing during daily occupation, so that the plant management is sometime critical - To ensure thermo-hygrometric comfort conditions and indoor air quality - To reduce energy consumption of plant system Dynamic simulation as an investigation tool to evaluate improvements through different system management strategies and technical solutions

New building of Faculty of Engineering, Bologna CASE STUDY New building of Faculty of Engineering, Bologna

CASE STUDY: THE CLASSROOMS

ENERGY MODELING: THE BUILDING Two adjoining thermal zones: same internal dimensions (L=14.0 x W=8.85 x H=3.50 m) maximum occupancy of 96 students —> internal loads external windowed façade (triple glazing for the lower fixed ribbon, quadruple glazing for the upper moveable ribbon, both with solar control properties in the outer glazing layer), oriented on north-west side and protected by a solar screen on the top of the building —> main external loads floor and opaque wall surfaces are bordering to internal conditioned areas, except south-east ones that are exposed to an unconditioned corridor

CASE STUDY: THE HVAC SYSTEM Air Handling Unit (AHU) serving the two classrooms: constant flow rate (8500 m3/h) air treatment sections with heating, cooling, vapor humidification, and post-heating return/external mixing box Single zone distribution: inlet air controlled by a thermostat detecting the temperature of mixing air extracted from both classrooms Energy supply: air condensing chiller for cooling water gas boiler for hot water TA05 TA06

ENERGY MODELING: THE HVAC SYSTEM - In the air loop, components not used in the cooling AHU configuration were neglected (heating and humidification sections) AIR LOOP CC HC (SP-HC) (SP-CC) CF OA EA TA05 TA06 D LEGEND CC cooling coil (SP-CC) cooling coil dew point temperature controller HC post-heating coil (SP-HC) post-heating coil temperature controller OAM outdoor air mixer EA exhaust air OA outdoor air TA05 TA05 classroom TA06 TA06 classroom D diffusers AC air chiller (SP-AC) air chiller temperature controller GB gas boiler (SP-GB) gas boiler temperature controller P pump - Due to complexity of water chiller and gas boiler plants, used also for other rooms and offices of the building, they are modeled in order to simply supply the cooling and heating coils with the power they need at each time step calculation COLD WATER LOOP HOT WATER LOOP AC P GB P (SP-AC) (SP-GB) CC HC

CALIBRATION OF THE MODEL - The model has been calibrated on the basis of the data collected during two days of measurements of external climate data, indoor comfort parameters, HVAC system operating parameters (timestep data capture = 5 min.), and recording the actual use of classrooms (occupancy level , use of window blinds, ...) - The calibration of the model was not easy to achieve because of uncertainties on some thermal characteristics involved in the building envelope and, in addition, because of the non-uniform temperature of air in rooms caused by a not perfect balancing of the air inlet jets - Due to our priority of energy analysis of the HVAC system, the model has been refined to achieve good correlation on internal temperatures and inlet temperatures

SIMULATION ON ACTUAL SITUATION (AS) Simulations have been carried out taking as basis actual HVAC system operating conditions, and guessing a weekly occupancy level, the same for both classrooms Basic settings parameters

TECHNICAL SOLUTION PROPOSALS: ONLY MANAGEMENT STRATEGIES ON ACTUAL HVAC SYSTEM reducing the amount of external air flow to the value of 7 l/s per person, the minimum required by reference national standard (UNI 10339) for university classrooms: the external air flow rate is than reduced to 60% of the total air flow increasing the condensation set point temperature of cooling coil at values ≥ 15°C: this case results an increasing of indoor humidity, but not above 60% in the case of maximum occupancy of each classrooms controlling the recirculated air flow fraction on the basis of the current occupancy level of the classrooms enabling free cooling in the first morning hours, depending on the current external air temperature (if ≤23°C) combining solutions S2, S3 and S4 S1 S2 S3 S4 C Changed settings parameters

TECNICAL SOLUTIONS PROPOSAL: AIR HANDLING UNIT RESIZING Reduced total air flow rate: 8500 m3/h —> 4850 m3/h New smallest fan, new less powerful cooling and post-heating coils Simulations with the new AHU are performed (new simulation starting point – NSP), and applying the same previous management solutions (respectively named S2R, S3R, S4R, CR). To increase energy savings TECNICAL SOLUTIONS PROPOSAL: MULTI-ZONE CONTROL The original concept of the HVAC control is modified Multi-zone solution, where the single post-heating coil on the AHU is replaced by two half-power ones, located just upstream the distribution air ducts of each classroom. A single thermostat for each classroom controls the power delivered from the corresponding post-heating coil, in order to maintain the internal set point at 26°C To achieve Indoor comfort conditions also with asymmetric occupancy levels

SIMULATIONS RUN PERIODS Typical cooling season , 15th May – 30st September, IGDG weather data - Effectiveness of menagement solutions is closely related to the climatic conditions against wich the HVAC system shall ensure indoor comfort conditions - Solutions that are effective during periods of high outdoor air temperature (and more effective as the temp. are high), turn out to be useless when the loads are moderate or even absent Energy saving of almost all solutions is as higher as the period is warmer. Results from simulations runned over the same week, but with the EPW climate data file built on the basis of measured 2012 data (detected by means of a weather station during the experimental measurement campaign), show improvements in energy saving, except in case S2 Whole typical cooling season A warm week, 25th June – 1st July, IGDG -VS- 2012 weather data Warm week

MANAGEMENT SOLUTIONS ENERGY CONSUMPTIONS SIMULATION RESULTS: MANAGEMENT SOLUTIONS ENERGY CONSUMPTIONS Over IGDG weather data - All four solutions are effective energy saving on a warm week, especially S3 where the high re-circulated air carry out a lower cooling coil energy consumption - Adding to this solution the advantages related to an higher condensation cooling coil temperature, and the free cooling, the best performance among management technical solutions is achieved (combination C of solutions from 2 to 4), up to 30% energy saving - On a seasonal simulation only S3 brings to some consumption reduction - Seasonal energy consumption of AHU components point out an improper design of the HVAC system: the high air flow rate value causes too high energy demands not only for the cooling coil but also for the post-heating coil

ENERGY CONSUMPTIONS WITH REDUCED TOTAL AIR FLOW RATE SIMULATION RESULTS: ENERGY CONSUMPTIONS WITH REDUCED TOTAL AIR FLOW RATE Over IGDG weather data - Energy consumption is cut down and, with reference to the starting point of actual situation, the savings are even greater, up to a maximum of almost 60% for the combined solution - With a reduced air flow rate more attention should be paid to the regulation of diffusers in the classrooms, so that the inlet air jet at a lower temperature (never falling under 18°C) does not cause conditions of local discomfort for the occupants

SIMULATION RESULTS: COMFORT EVALUATION Trend of temperatures in a typical warm day SINGLE-ZONE CONTROL MULTI-ZONE CONTROL - Different occupancy level: TA05 100% TA06 10% - The actual configuration of HVAC system can achieves comfort conditions in both classrooms only when they have the same occupancy level. However a different number of students is very often present in the two classrooms - The replacement of the AHU fan could bring to energy savings, but would not improve HVAC system performance in terms of internal comfort when an asymmetric occupancy occurs: mean air temperature differences between the two zones higher than 2 °C, that produce little discomfort in both classrooms - The HVAC configuration with the two separated post-heating coils would be able to ensure comfort conditions even for asymmetric occupancy

CONCLUSIONS Dynamic simulations as an important tools for the energy analysis of building-plant systems not only in design process but also in management decision for improve energy saving in existing buildings. In the case study of a typical HVAC plant designed for hygro-thermal and air quality control in university classrooms located in Bologna, energy simulations gave important information for the best setting of the air handling unit and for the evaluation of energy efficient solutions. Energy saving solutions must be evaluated depending on the effective external climatic conditions and daily scheduling of HVAC system. Problems of the existing plant are related to excessive air renovation causing high energy consumptions and a lack of control of internal temperature of two classrooms treated as a single zone, creating a discomfort when different occupancy of two rooms occurs. The main and simplest solution is the increase of the recirculating air flow rate (as for S1 and S3 solutions) that is an efficient strategy only if it is directed to reduce cooling loads. While in warmer summer periods this solution give high energy saving, less advantages are present in a seasonal analysis. Other solutions analysed are the increasing of the cooling coil dew point temperature (controlling the internal humidity) and reducing the volume total flow rate. For all those solutions, simulation results indicate up to 60% energy savings for a typical summer week in Bologna.

Giovanni Semprini1, Cosimo Marinosci2 and Alessandro Gober3 THANK YOU FOR YOUR ATTENTION Giovanni Semprini1, Cosimo Marinosci2 and Alessandro Gober3 1DIN - CIRI, University of Bologna, Bologna, Italy, giovanni.semprini@unibo.it 2 DIN - CIRI, University of Bologna, Bologna, Italy, cosimo.marinosci@unibo.it 3 CIRI, University of Bologna, Bologna, Italy, alessandro.gober@unibo.it