# Improved Solar Operation Control for a Solar Cooling System of an IT Center
## 1 Introduction  
Worldwide, an increase of the cooling demand is expected. In the building sector, the cooling demand is the most rapidly increasing effective energy demand [1]. Usually, the cold generation is realized by electrically driven compression chillers. Hence, the expected development in effective energy demand will lead to an increased power requirement as well. Acting on the assumptions of the World Energy Outlook [2], the global power requirements will increase by 60% between 2016 and 2040. Approximately 15% to 20% of this elevation will be caused by the cooling demand. Consequently, solar-assisted cooling systems (SAC systems) have a large potential to reduce the fossil fuel consumption related to non-regenerative power generation. On the other hand, it has been shown (e.g., in [3]) that a minimum solar fraction is necessary for SAC systems in order to achieve a lower primary energy consumption than a conventional system using an electrically driven compression chiller. [PERSON] et al. [3] illustrated that the system performance with regard to primary energy savings is improved when the COP of the thermally driven chiller increases, the solar fraction increases, and the specific electricity consumption of the auxiliary components like supply pumps and cooling tower etc. decreases. For SAC systems with parallel backup heating, the achievable solar fraction is a question of the possible operating time with solar heat from the collector. Solar heat operation (SHO) can start earliest when the collector temperature is higher than a minimum driving temperature \(\mathrm{t}_{\mathrm{D}}^{\mathrm{min}}\) and has to end latest when the collector temperature drops below. Of course, additional temperature differences have to be taken into account (e.g., for a heat exchanger between the collector and storage, heat lossesetc.). The minimum driving temperature \(\mathrm{t}_{\mathrm{Di}}^{\mathrm{min}}\) depends on the cooling load as well as the capacity and part load behaviour of the chiller. Moreover, it depends on the possible cooling water temperature, which is limited by the dry or wet bulb temperature. Generally speaking, \(\mathrm{t}_{\mathrm{Di}}^{\mathrm{min}}\) depends on the load and meteorological conditions as well as the technology used for the chiller and the reject heat device but not on the collector technology. Nevertheless, the collector technology (including storages and the connection to parallel or serial backup heating systems) has a large influence on the necessary preheating time (i.e., the period from sunrise until \(\mathrm{t}_{\mathrm{Di}}^{\mathrm{min}}\) is reached and solar heat operation can start) and thereby influences the overall system performance [4]. Taking the heat capacity effects of the collector and installation, as well as heat losses to the surrounding into account, [PERSON] [5] determined a solar radiation threshold \(I_{min}\) as an equivalent measure to the minimum driving temperature. For a small SAC system in Madrid, two different heat rejection technologies were compared for the same meteorological conditions. In comparison to a wet cooling tower, a dry cooler revealed approximately 12 K higher cooling water temperatures. This led to about 20 K higher driving temperatures in the collector circuit and reduced the possible operating time of the SAC system from 9 to 3 h. The preheating time increased from approximately 3 to 6 h.  
The influence of heat capacity effects on the preheating time was investigated by [PERSON] [6] and [PERSON] [7] as well. While [PERSON] used a separated smaller part of a partitioned hot water storage tank to reduce the preheating time, [PERSON] applied different temperature nodes of the storage tank for the control of the chiller before, during, and after solar heat operation. In both references, a constant (i.e., load and weather independent) start-up temperature of \(\mathrm{t}_{\mathrm{Di}}^{\mathrm{min}}=75\) and 60 \({}^{\circ}\)C was used.