Humidity Performance in Summer

6.1Simulation Study of Cooling Concepts

The simulation model is a new three-floor two-wing office building and contains two office-rows with a dimension of 5.2 m in length, 3.9 m in width, and 3.0 m in height for each office which are separated by a corridor (width 2.6 m). The building is simulated in North-South and East-West orientation.

The simulation model represents a typical European office building with an area-to-volume-ratio of 0.4 m²_ext.surface/m³_int.volume and a window ratio of 0.32 m²_window/m²_ext.wall. The building physical properties meet the EPBD requirements:

1. 
   external walls, baseplate, and ceiling: U_mean=0.24 W/m²K incl. thermal heat bridges
   
2. 
   windows: U_w=1.0 W/m²K and g=0.58
   
3. 
   solar shading: external Venetian blinds (F_c=0.06, F_c=0.2 considering non-optimal closing) are closed semi-automatically when the solar radiation on the façade exceeds 200 W/m²
   

Figure 5 Building simulation model: Typical European office building for North-, Mid- and South-European countries.

Figure 5 Internal heat gains during working days in summer period: The internal heat gains from artificial lighting differs from month to month and with the latitude.

Plant Model and cooling concepts

Five different cooling concepts are applied to cool the office building (Figure 5 ). All concepts allow for free ventilation by window opening. Four concepts employ exhaust fans to provide a minimum air change rate of 40m³/h per person. Though in actual projects an exhaust and supply air system may be applied to pre-heat the air in winter by a heat recovery system and/or to dehumidify the supply air in summer, the simulation study considers only an exhaust air system for better comparison of the sensible cooling capacity of low-energy cooling concepts.

1. 
   Passive cooling refers to techniques used to prevent and modulate heat gains. The reduced cooling loads can be dissipated by free ventilation only. The building has external shading to avoid overheating. A bare concrete ceiling modulates the internal and solar heat gains. Open windows during the night enable an increased single-side and cross ventilation.
   
2. 
   In the investigated office building, the air change rates differ from day to day and from location to location. The air change exceeds often 2 h^-1 in the cooler summer climates, while the warmer summer nights in South Europe allow for maximum air change rates of 1.8 h^-1.
   
3. 
   An exhaust ventilation system can also be used for mechanical night ventilation and provides an air change rate when the room temperature exceeds 21 °C with a minimum temperature difference between inside and outside of 2 K.
   
4. 
   A fan coil unit is simulated as a reference system. The fan coil unit provides sensible cooling to meet the comfort requirements during the time of occupancy. A compression chiller provides cold water with a supply temperature of 13 °C. The design return temperature is 18 °C. A cooling tower is used for re-cooling. The maximum cooling capacity is limited to 1.8 kW or 90 W/m², respectively.
   
5. 
   The coefficient of performance COP decreases from North to South due to increasing ambient air temperatures during the time of operation. The mean COP_mean during the time of operation decrease from 3.1 in Stockholm to 2.4 kW_cooling/kW_el in Palermo.
   
6. 
   A radiant cooling ceiling panel is operated during the time of occupancy. Its cooling capacity is a function of the difference between the mean cold water temperature and the room temperature. For a typical temperature difference of 8 K the specific cooling capacity is approx. 100 W/m². The panel covers 70 % of the office area which results in a cooling capacity of 70 W/m² for ΔT=8K. The actual maximum cooling capacity in Milano is 77 W/m² for ΔT=9K. The supply temperature is controlled by the equation T_supply[°C]=18°C + 0.35 (18°C – T_ambient[°C]) with a minimum supply temperature of 16°C to avoid condensation.
   
7. 
   A bore-hole heat exchanger is used as heat sink. The undisturbed ground temperature in summer is calculated for each climate zone and increases from 6.3 °C in the North to 19.6 °C in the South.
   
8. 
   If the return temperature from the bore-hole heat exchanger exceeds the set temperature, an optional compression chiller will provide additional cooling. As the bore-hole heat exchanger in Stuttgart provides cool water during the whole summer, the seasonal energy efficiency ratio SEER is 14 kWh_therm/kWh_el. In Rome, active cooling is needed and, hence, the SEER is 3.4 kWh_therm/kWh_el only.
   
9. 
   A concrete core conditioning (TABS = thermo-active building system) cools the whole ceiling during the night. Due to the high thermal inertia, the mean cooling capacity of approx. 40 W/m² is provided during the whole day. This results in a considerable fluctuation of the room temperature during the time of occupancy.
   
10. 
    The control strategy is similar to the operation of the radiant panel but with night-time operation. The seasonal energy efficiency ratio SEER is 14 kWh_therm/kWh_el in Stuttgart, too. In Rome, the SEER is 3.8 kWh_therm/kWh_el due to higher supply temperatures than for the operation of the radiant cooling ceiling panels.
    

Figure 5 Five different cooling concepts: Passive cooling, night ventilation, active cooling with compression chiller, and water-based low-energy cooling (with compression chiller when needed to meet the cooling load). Green: ventilation, blue: cooling, and grey: heat sink.

investment costs

The investment costs are calculated for the typical office building shown in Fig 7-1. These cost estimations are based on an analysis of realized HVAC concepts in Germany [Voss et al. 2006] and [BBR 2007]:

1. 
   Passive cooling: 20 €/m². Ventilation slats and enlarged openings for a lower pressure drop.
   
2. 
   Mechanical night ventilation: 32 €/m². Ventilation slats, exhaust ventilator, and ducting. Control system.
   
3. 
   Fan coil: 85 €/m². Ventilation slats, exhaust ventilator, and ducting. Compression chiller with cooling tower, fan coil units, and cold-water piping. Control system.
   
4. 
   Radiant cooling ceiling panel: 138 €/m². Ventilation slats, exhaust ventilator, and ducting. Compression chiller with bore-hole heat exchanger, suspended radiant cooling, and heating panel and piping. Control system.
   
5. 
   Thermo-active building system: 117 €/m². Ventilation slats, exhaust ventilator, and ducting. Compression chiller with bore-hole heat exchanger, concrete core conditioning, and piping. Control system.
   

The simulation study is carried out for 6 different European climate zones. Each climate zone is defined by the mean ambient air temperature in August and is characterised by a meteorological reference station. The summer temperatures stay below 16 °C in Stockholm, between 16 and 18 °C in Hamburg, between 18 and 20 °C in Stuttgart, between 20 and 22 °C in Milano, between 22 and 24 °C in Rome, and exceed 24 °C in Palermo. Note: These summer climate zones correspond reasonably with the USDA Hardiness Zones from 5 (in the North) to 10 (in the South).

Building and plant simulation

The coupled building and plant simulation is run for the summer period from May to September. The cooling load is calculated for both the static comfort model according to ISO 7730 and the adaptive model according to EN 15251. The cooling capacity and the end energy use for cooling are calculated only for the operative room temperature according to the adaptive comfort model.