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Space cooling demands in districts

Due to increasing comfort requirements and more pronounced heat waves in summer, steadily rising cooling demands for space cooling are expected in the future.

Space cooling

In commercial buildings, such as office buildings, the installation of systems for air conditioning has become more and more standard. Especially for businesses with customer contact, air conditioning is a key factor in increasing customer satisfaction. Banks, hotels or shopping centers, for example, must have air conditioning. Shopping centers in particular have high internal loads, which leads to considerable cooling loads in summer but also in the transitional period. In the residential sector, on the other hand, air conditioning is still rarely encountered, and when it is, it is often implemented via decentralized mono-split units. With increasingly frequent and more pronounced periods of heat, as well as an increased need for an appealing living comfort, the demand for air conditioning solutions is becoming more and more important. Large glass facades in new buildings also increase the heat loads in summer. With the move away from fossil heat generators (gas or oil boilers) to low-temperature solutions, especially heat pumps, there are new synergy effects for the provision of heating and cooling in the residential sector.

What technologies are available for air conditioning?

In buildings, a number of different technologies are used for air conditioning. Monosplit or multisplit units are very common. These function like inverted heat pumps, i.e. heat is transferred indoors to a heat carrier fluid, which then undergoes compression. The resulting increase in temperature of the heat carrier fluid allows the heat to be released into the environment in the outdoor unit. The heat carrier fluid is then expanded again, reducing its temperature so that it is below the temperature of the room to be air-conditioned and can draw heat from the room again. Mono-split units have only one indoor unit (e.g. for one room), which is coupled with an outdoor unit. In multi-split units, several indoor units extract heat from the rooms. The heat carrier fluid from all indoor units is then transported to a central outdoor unit, where the heat is released to the environment. For large central systems, there are often one or more outdoor units (recoolers) that supply cooling to a large number of indoor units. The indoor units can be wall-mounted, facade-mounted or ceiling-mounted.

How is the space cooling demand of a building determined?

For the calculation of the space cooling demand, all heat flows of a building are balanced, as in the case of the space heating demand calculation:

  • \(Q_T\): Transmission heat flows due to heat conduction (through walls or windows)
  • \(Q_V\): Ventilation heat flows as a result of air exchange (through leaks and ventilation)
  • \(Q_i\): Internal heat loads (operation of electrical devices, lighting or body heat)
  • \(Q_s\): Solar heat gains (solar irradiance through windows)

In contrast to the heat demand calculation, the internal heat loads and solar heat inputs increase the cooling load. For shopping centers with large glass facades and high customer density, this results in particularly high cooling loads. The relevant thermal flows of a building can be modeled with the help of dynamic simulation models. Alternatively, simplified methods can be used - analogous to the calculation of heating demands - such as the degree-day method. In this methodology, a linear relationship is assumed between the cooling demand and the temperature difference between the interior and exterior air: $$Q_c \propto (T_a - T_i)$$ Accordingly, the maximum cooling load occurs at the maximum outside air temperature (design temperature). The cooling load of a building is usually given in W/m². Experience-based full load hours can also be used to calculate the annual cooling demands. For office buildings in Germany, these are around 500-1000 h/a. The full load hours \(\tau_V\) of the air-conditioning technology can be expressed as follows: $$\tau_V = Q_c^{tot} / Q_c^{max}$$ in which \(Q_c^{tot}\) is the annual cooling demands in kWh and \(Q_c^{max}\) the maximum cooling load in kW.

Cooling demands for different room types

In the nPro tool, typical area-specific cooling demands are available for a large number of building types.
Table 1: Space cooling demand for different building types, from [1]
Room type Cooling load
Residential 30-40 W/m³
Office 50 W/m³
Department stores 50-60 W/m³
Glass extensions 200 W/m³

Table 1 lists the volume-specific cooling loads for 4 room types. While residential rooms have a cooling load of about 30-40 W/m³, the cooling load for offices is higher at about 50 W/m³ due to the high internal heat loads (IT infrastructure, lighting, high occupancy density). Retail spaces have an even higher cooling load of 50-60 W/m³, since internal heat loads have a major influence here (lighting and high occupant density) and large glass facades are often installed. Cooling loads of 200 W/m³ can occur in glass extensions.

Sources

  1. Planungshandbuch Wärmepumpen (Viessmann)
  2. Fachbuch: Recknagel - Taschenbuch für Heizung + Klimatechnik 2017/2018

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