The system configuration and sizing depend on the load profile and the available space of the building. There are different system configurations, such as direct or indirect, open or closed, active or passive, and single or multiple loop, each with their own implications for the fluid type, pump size, piping layout, and heat exchanger design. For example, direct systems use water as the fluid and circulate it directly from the collectors to the storage tank or the load, while indirect systems use a heat transfer fluid (such as antifreeze) and circulate it through a heat exchanger to transfer heat to the water. Open systems are exposed to the atmosphere and can be affected by freezing and corrosion, while closed systems are sealed and pressurized and require expansion tanks and safety valves. Active systems use pumps and controllers to move the fluid and regulate the temperature, while passive systems rely on natural convection and thermosiphon effects. Single loop systems have one fluid circuit that connects the collectors, the storage tank, and the load, while multiple loop systems have separate fluid circuits for each component. The system sizing should be based on the peak demand and the solar fraction of the building. The peak demand is the maximum amount of hot water or space heating required at a given time, and the solar fraction is the percentage of the load that can be met by the solar thermal system. The system sizing should balance the trade-off between the collector area, the storage volume, and the backup heating source.