Presentation on theme: "LOW ENERGY COOLING and THERMAL COMFORT"— Presentation transcript:
1 LOW ENERGY COOLING and THERMAL COMFORT Establishing an annex for EPBD-related CEN-standards for buildings with high energy efficiency and good indoor environmentLOW ENERGY COOLING and THERMAL COMFORTJarek Kurnitski, D.Sc., Adjunct Professor Sitra, the Finnish Innovation Fund Helsinki University of Technology, HVAC-Technology
2 ObjectivesLow energy cooling can mean limited capacity cooling (groundwater, earth to air, slab cooling, night ventilative cooling, etc passive) which is somewhere in between air conditioning and free running buildings not addressed in the standards (EN 15251, ASHRAE 55)EN deals with buildings which either have mechanical cooling (full cooling capacity, PMV index) or buildings without mechanical cooling systems (adaptive approach)ThermCo project:evaluated how do people perceive “limited cooling capacity”assessed the applicability of adapted approach for such buildingsformulated recommendations for revision of EN regarding thermal comfort criteria for buildings with low energy cooling in summerprepared REHVA guidebook on low energy cooling
3 Background of the project Large German study (Pfafferott et al. 2007) with 12 low-energy office buildings with ‘‘mixed-mode systems’’ such as hybrid day ventilation, enhanced night ventilation, earth-to-air heat exchanger and TABS with ground coolingSuch buildings can show low energy use, but they don’t keep temperature (especially with earth-to-air) due to limited cooling/ventilation capacity and controlCan adaptive thermal comfort model used in such buildings?
4 PMV model EN 15251, ISO 7730, CR 1752, ASHRAE 55 a cornerstone of thermal comfort theory leading to target values to be achieved with heating/ cooling season design temperatures depending on indoor climate category (activity & clothing)Includes behavioral adaptation through clothing value (1.0 clo in winter/0.5 clo in summer)
5 Adaptive model EN 15251, ASHRAE 55 For free running buildings with openable windowsArguably people have lower expectations in such buildings and adapt to accept higher temperatures (productivity data is not available from the adaption studies)
7 Use of the adaptive approach ThermCo project did not established any conclusive evidence on the use of adaptive approach in buildings with limited cooling capacity/controlIt was concluded that the same expectancy can be expected as in mechanically cooled buildings, i.e. PMV-model shall be usedIt was also proposed that the PMV model has to be used for sizing of the room conditioning units, but the system capacity may be undersized according to adaptive model, but there was no scientific evidence available supporting this (also no evidence against that exists)It can be expected that some (minor) adaptation to outdoor temperature can be seen in every building with room controllers (non-centrally controlled, non- overcooled buildings), but no evidence existsAs no evidence exists one shall use PMV-model.
8 Energy saving doesn’t always mean energy performance increment Relative performance as a function of temperature REHVA Guidebook 6 (2006)
9 Recommendations for revision of EN 15251 The recommendation is to write a guideline in the standard saying that:some cooling is better than no cooling and if a system with limited cooling capacity is installed the designer must shown how often the room temperatures is expected to exceed the temperature limits specified for mechanical cooled buildings in EN15251and also show this for the adaptive approach.This will provide the client with data for the expected performance.It is stressed that the intention of the standard should not be that a building must be in one category the whole year; but rather use the categories to provide a footprint of the performance by showing the % of occupied time in each category. This should also be made clearer in the standard
10 Limited cooling/ventilation capacity/control Report % of occupied time in each category to the client …
11 Revision of EN 15251/ the season change The season change from cooling to heating season should be clearly addressed15C outdoor temperature should be used for season change independently of direction from heating to cooling or cooling to heatingIn many offices there may be significant long periods with cooling demand in winter, but the clothing level chosen by people may be winter levels as the outside temperature is low. So the criteria is an upper limit for the heating room temperature of C depending of the category chosenIt is also addressed that the present formulation of adaptive model may result in some climatic zones lower room temperatures required for the buildings without mechanical cooling. (As the criteria for buildings without mechanical cooling vary with outside temperatures in summer, but criteria for mechanical cooled buildings stay the same for the summer.
12 Lower temperatures required for free-running The problem disappears when the season change limit is corrected to 15C
13 Low energy cooling REHVA-guidebook As there are many low energy cooling solutions (including passive and mechanical cooling) not compromising air quality and thermal comfort if properly implemented it was decided to prepare REHVA-guidebookGuidebook will includeDefinitions of seasonal performanceGuidance on thermal comfort criteriaDescription and examples of technical solutions
14 Seasonal performance system boundaries Example of Aachen (Germany) office building: Whereas the heat pump itself has a COP of 4.8 the system efficiency over the whole heating period, expressed here as a "seasonal performance factor (SPF)", varies between 12 and 2.6 depending on the system boundaries (and the corresponding auxiliary energy for pumps) – same applies for the cooling
15 Energy efficiency of cooling Seasonal COSP (called also SEER, SCOP or SPF) should be estimated by energy simulation calculation. Seasonal COSP is a weighted average COSP for a year.Manufactures give performance data at full load according to EN :2007 and at part load according to prEN 14825:2008 (ESEER, the seasonal value SEER for Europe)EER values include the compressor unit (effective power input definition of EN :2007). The values do not include the condenser fans (despite of integral condenser in the liquid chilling package) and the distribution system, so the application seasonal ESEER value has to be recalculated for the distribution system used in the real building.
16 Calculated exampleSimulation example for the building with conventional cooling (COP=2.7):Delivered cooling energy by plant is 42.8 kWh/(m2 a) consisting of:29.7 kWh/(m2 a) to chilled beams (with central water distribution system)and 13.1 kWh/(m2 a) to cooling coils of air handling units. Electrical energy use of cooling system is 9.5 kWh/(m2 a), consisting of:8.6 kWh/(m2 a) electricity use of compressor unit0.6 kWh/(m2 a) electricity use of condenser fans0.3 electricity use of circulation pumps.the application seasonal SEER is 42.8/9.5 = 4.5
17 Read also ThermCo article on high temperature room conditioning solutions: REHVA Journal, August 20091. Chilled beams (water radiators for heating)2. Ceiling panels3. Floor cooling and heating (pipes in the slab)Additional features (not included in basic cases):demand controlled ventilationnight ventilationshorter chiller operation time / operative temperature control