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[Translate to English:] liNear Building Cooling Dynamic

Dynamic Cooling Load Simulation according to „ASHRAE“: A physical approach with simple operation

By means of Building Cooling Dynamic, it is possible to dynamically determine the cooling load. This dynamic calculation method simulates the temperature development in the room by the use of the profile of a design day. It does not only allow the calculation of the cooling load with predetermined target temperatures, but also the calculation of the cooling load at a limited cooling capacity (Which room temperature can still be reached at a preset capacity?). Furthermore, it is possible to calculate the room temperature completely without any cooling (free-running temperature).

The dimensioning an energy-efficient system requires that the calculated cooling load corresponds not only approximately but exactly as possible to the real cooling load. The input and the calculation time in the calculation programs should be as minimal as possible in order to ensure efficient processing. According to this area of conflict, various cooling load calculation methods have been developed; amongst others acc. to VDI guideline 6007 or VDI guideline 2078 [1], Ö-Norm H 6040 [2] and acc. to ASHRAE [3] [4]. The following article presents the worldwide accepted and validated method by ASHRAE applied for many years in the physical room and building simulation.

Heating and cooling load calculations form the basis for the dimensioning of heating and cooling systems. They affect the size of pipes, air ducts, outlets, radiators, boilers, refrigerators and all other components that are needed for the indoor air-conditioning. Therefore, they have a great impact on the construction and operating costs of the building, the energy consumption, comfort and thus also on the productivity of users. Especially in times of rising energy costs, a precise calculation of large and even small systems makes a significant contribution to the capital and operating cost optimization. Meanwhile, the operating costs are to be in the focus of user interest. The principle ‘more is better' is replaced by the requirement of a precise energy input.

 

About ASHRAE

The 'American Society of Heating, Refrigerating & Air-Conditioning Engineers', ASHRAE for short, is an engineer association similar to the VDI. In the field of building services and systems, they made their business developing sustainability, energy efficiency and air quality in terms of heating, cooling and air climate technology for the industry.

The software company liNear located in Aachen has implemented the development of the dynamic cooling load calculation method with the approach according to ASHRAE to provide a suitable software solution due to numerous requests from international markets. In addition, it can be applied as an alternative to VDI 2078. The cooling load method according to ASHRAE is already being applied in building simulation programs such as 'TRNSYS' [5] and 'Energy Plus' [6]. Comparative measurements show that this model corresponds to the reality. In the ASHRAE Research Project RP 1117 [7], an analogy was detected by measurements using real rooms.

Below, the differentiated cooling load method according to ASHRAE, Non-Residential Cooling Load [8] [4] is presented, which is used worldwide in science and practice and is implemented in the cooling load simulation program ‘liNear Building Cooling Dynamic'.

 

Accurate thermal modeling of building androom enveloping surfaces

The requirements for a physically correct dimensioning are met by ASHRAE with the solution of the Fourier heat transfer equation [9].

Thus, the heat transfer equation of the walls with their geometric and thermophysical properties, taking into account previous system states, (delayed heat storage and release in walls) using the 'State-Space Method' is directly dissolved. This approach results in extensive possibilities, like the simulation of the room temperature (e.g. a free-running temperature), limited cooling capacity requirement and requirement free of temporal restriction of internal and external loads (Ill. 1). Moreover, the operating time of the system can also be restricted. In addition to the internal loads such as lighting, machinery, devices and individuals also the material throughput and the infiltration of outside air or air flow from neighboring rooms can be considered. Depending on the temperature of the air that enters via infiltration into the room, or the temperature of another substance, these factors can positively affect the cooling load as well; namely having a cooling effect. There are numerous ways to determine the temporal heat flows in the wall. Within building simulations the application of CTF coefficients (Conduction Transfer Function) has been established, which is known from the analysis of systems using the State-Space-Method [10] [11]. The heat transfer equation of the wall is converted into a heat transfer system of differential equations and solved by a defined time step. After transformations, a series of finite CTF coefficients results from the State-Space-Method. These coefficientsenable the calculationof the currentspecific heat flow rates andthe current temperatureson the inside andoutside of eachenveloping surfaceon the basis of the past temperaturesand heat flow rates of the lasthours (Ill. 2). The more heavy the wall, the more coefficients must be taken into account from the past. The cooling load method according to ASHRAE allows the output of the internal and external temperatures of the surfaces of each enveloping surface and for each time step. The thermal behavior in the room is highly dependent on the properties and condition of the enveloping surfaces. The enveloping surfaces can be provided with physical material properties (thermal conductivity, heat capacity, density, thermo-optical properties). On both sides of the wall time-varying loads are being induced into the wall. These factors include air temperatures, solar radiation, internal heat radiation exchange and convective processes.

 

Shortwave solar radiation

In contrast to the heat load, also components within the room are relevant for the calculation of the cooling load (room type). All enveloping surfaces of the room contribute to absorb the radiative part of the radiation and temporally delayed release again the absorbed and stored energy. The amount of direct and diffuse radiation which affects the enveloping surfaces and enters through the window into the room is determined by the location and its weather data. The Solar heat gain coefficient and the internal and external shading are additionally considered in windows. The weather data is provided by a division of the US Department of Energy (DOE) (Ill. 3). Thus, the data from 2,100 weather stations from all continents are available. In addition to the cooling and heating-design-day many more parameters for a physical examination are included in the weather data (such as coordinates, air pressure, outside air and dew point temperatures, optical depth of the atmosphere). Based onthe solar constant of 1367 W/m² up to the surface, the solar radiation is influenced by the following (Ill. 4): Due to the varying distance between the earth and the sun, the result will be adjusted by up to +/- 3.3% depending on the annual day. The solar radiation is divided into direct radiation, diffuse radiation from the atmosphere and diffuse ground reflectance. In this regard the sun height, azimuth, inclination and orientation of the building parts are playing a role. Furthermore, the optical depth for diffuse and direct radiation is applied for reduction of the radiation concerning transmission through the atmospheric layers till the ground. Since the solar radiation is an essential contribution to the heat input of a room, an editor for external window shading was developed. With this, different shading situations can be created and assigned to the windows. The shadowing configurations are shown three-dimensionally with a simulation of the sun radiation and any modification is directly visually comprehensible in fast motion. For each time step the program calculates the shadow portion for any external shading element by means of a raytracing model (Ill. 5). Thethermo-opticalpropertiessuch as groundreflectance, absorbanceandg-value determinethe absorption of thesolar radiationheatinto theenveloping surfaces.For internal shading control a global radiation sensor is provided.

 

Long-wave radiation model

The radiation model in the room is described by means of the Radiosity matrix procedure [12]. The emission, absorption, reflectance and mutual visibility of surfaces within the room plays a decisive role. Due to the realistic radiation models (Ill. 6) the system boundaries of the walls are represented by the internal and external surfaces. The physical approach allows precise conclusions regarding the temperature development in the room. In many other cooling load methods the heat loss by radiation is considered in a simplified way by using mathematical linearization. The result is that the heat transfer coefficient is only adequately applicable at a temperature near 20 °C. The deviation the heat flow rates between the two approaches is the higher, the more the surface temperatures differ from 20 ° C [13]; e.g. in the case of a free-running temperature. Concerning the external surfaces, the environment is taken into account according to their inclination. The view factor is divided in the proportions of the earth and the sky. Furthermore, the exchange of radiation depends on the emission coefficient of the wall. In the process, the ground surface temperature is set equal to the air temperature. The long-wave heat radiation outside and inside the building is calculated by means of the diffuse radiation model according to Stefan-Boltzmann.

 

Conclusion

The extensive simulation approach of dynamic cooling load calculation according to ASHRAE allows valid results also in special applications, e.g.:

 

  • Calculation of the cooling load with limited cooling capacity (e.g. CCT panel cooling),
  • Calculation of the free-running temperature (no cooling capacity),
  • Time limitation of system operation by usage profile,
  • Night cooling by the supply of outside air,
  • Calculation of the surface temperatures of each enveloping surface,
  • Window shading simulations,
  • Active window shading through external global radiation sensor,
  • Compatibility of temporal individual internal loads by usage profiles,
  • Moisture balance and control of humidity

 

Due to the general physical heat balancing approach, the method can be extended by any technical features and control scenarios. The development has only just begun, but the software application already enables future-oriented methods for the planning and dimensioning of cooling devices. Along with a physically accurate cooling load calculation and various application scenarios, it is offered the possibility, in Germany and worldwide, of sustainable cost savings by avoiding over-dimensioning by means of a precise energy input.

 


 

List of references

[1] ASHRAE: American Society of Heating and Refrigerating Engineers
[2] ASHRAE Fundamentals 2013 (SI Edition)
[3] ASHRAE Fundamentals 2013 Chapter 18, Nonresidential Cooling and Heating Load Calculation
[4] ÖNORM H 6040 (2012) Berechnung der sensiblen und latenten Kühllast von Räumen und Gebäuden
[5] VDI 6007/ VDI 2078 (2015)
[6] Modelling of Heat Transfer in Buildings, Dissertation, J.E. Seem, 1987
[7] Calculating building heating and cooling loads using the frequency response of multilayered slabs, Dissertation, D.C. Hittle, 1981
[8] TRNSYS, Transient Simulation Tool (University of Wisconsin, Transsolar, TESS.)
[9] EnergyPlus 8.4, Building Energy Simulation Programm (DOE, Department of Energy, USA)
[10] ASHRAE Research Project RP 1117 (2013): Experimental Validation of Design Cooling Load Procedures: The Heat Balance Method, D.E. Fisher, C. Chantrasrisalai, I.Iu, D.S. Eldridge
[11] DIN EN ISO 6946 (2008), Tabelle A.1, Temperaturabhängigkeit Wärmeübergangskoeffizient
[12] Wärme und Stoffübertragung (2003), 4. Auflage, H.D. Baehr, K.Stephan

 


 

(Bild 1) Programmoberfläche der Kühllastberechnung nach ASHRAE und Ergebnisdarstellung der inneren Lasten
[Translate to English:] (Bild 1) Programmoberfläche der Kühllastberechnung nach ASHRAE und Ergebnisdarstellung der inneren Lasten
Bild_2_-_Wandsimulation.png
[Translate to English:] (Bild 2) Ergebnisse einer Simulation von Oberflächentemperaturen
Bild_3_-_Klimadaten.png
[Translate to English:] (Bild 3) Weltweite Wetterdaten stehen kostenfrei u. a. für die physikalisch korrekte Simulation des Strahlungsmodells zur Verfügung.
Bild_4_-_Visualisierung-Strahlungsfluss.jpg
[Translate to English:] (Bild 4) Modell der solaren Strahlung
Bild_5_-_Verschattungskonfigurator.png
[Translate to English:] (Bild 5) Verschattungskonfigurator mit dreidimensionaler Darstellung der Sonnenstrahlensimulation im Zeitraffer
Bild_6_-_Modell-Netzwerk-Waermeaustausch.png
[Translate to English:] (Bild 6) Netzwerk des konvektiven und radiativen Wärmeaustauschs (vereinfachte 2D-Darstellung in der Draufsicht; Berechnung wird im dreidimensionalen Raum durchgeführt)