Solar thermal systems

Diagram of solar thermal heating system. (Source: https://www.researchgate.net/figure/Diagram-of-solar-heating-system_fig2_327680052)

Solar thermal systems capture heat from the sun to provide:

• Service hot water
• Space heating

The hourly hot water generation, based on hourly solar radiation from the selected weather file, can be used to offset the domestic hot water demand or be stored for later use.

These systems are distinct from photovoltaic (PV) systems, which convert sunlight into electricity

How solar thermal systems work

The core of a solar thermal HVAC system involves:

1. Collection: Solar thermal collectors—like flat-plate or evacuated-tube collectors—absorb solar radiation. These collectors contain a heat transfer fluid (often water or an antifreeze solution like propylene glycol).
2. Heat Transfer: The fluid, heated by the sun, is circulated through the system.
3. Storage: The thermal energy is typically transferred to an insulated storage tank (usually water) for later use. This is a key advantage, as the heat can be stored for night or cloudy days.
4. Distribution (Heating): For space heating, the stored hot water is circulated through a heat exchanger to warm air in a forced-air system, or it's pumped directly through radiant floors or baseboard radiators. A conventional backup system is usually integrated for periods of insufficient solar heat.
5. Distribution (Cooling): For solar cooling, the high-temperature solar heat powers a thermally driven chiller, most commonly an absorption chiller. This process uses heat, rather than electricity, to drive a refrigeration cycle that produces chilled water for the building's air conditioning system

Key Design Parameters

Designing an effective solar thermal HVAC system requires careful consideration of several factors:

Solar Resource & Climate

• Solar Radiation Intensity: The amount of sunlight available at the location is paramount for sizing collectors.
• Climate: The local temperature range and likelihood of freezing dictate the choice of collector (e.g., evacuated tubes for colder climates or higher temperatures) and the type of heat transfer fluid (e.g., glycol for freeze protection).

System Sizing & Orientation

• Collector Area: The total area of the collectors must be sized to meet a significant fraction of the building's heating and/or cooling load.
• Collector Tilt and Azimuth: Collectors must be correctly oriented—typically facing the equator (south in the Northern Hemisphere) and tilted at an angle based on the site's latitude—to maximize solar gain, particularly during the season of peak demand (e.g., steeper tilt for winter heating).
• Thermal Storage Volume: The size of the hot water storage tank is crucial for storing heat to cover non-sunny periods, ensuring reliable operation.

Energy Demands

• Heating and Cooling Load: Accurate calculations of the building's energy needs, including domestic hot water, space heating, and cooling requirements, are essential for determining system capacity.
• Target Temperature: The required operating temperature influences the collector type and overall system efficiency (e.g., cooling via absorption chilling requires higher temperatures than simple space heating).