The performance of ground coupled heat pump systems with borehole heat exchangers can be significantly improved by unglazed solar collectors. In this combination, collector operation temperatures below the ambient air level and hence additional condensation heat gains occur. For an investigated and monitored system in Limburg, Germany, the condensation yield was determined to 3.7% or 19 kWh/(a m²). Thereby, condensation shows a significant dependency on the season. During the summer months, only 0.8% of the total collector yield is induced by condensation, while in winter it increases to 13%. The implementation of an established condensation model to the collector model according to EN 12975 is demonstrated. As main result the investigated the heat pump system performance is only marginally improved due to the condensation heat gains.

Unglazed Photovoltaic Thermal (PVT) collectors provide very high efficiency for heat and electricity generation at low temperatures. Therefore they are particularly suitable to support the heat source of heat pump systems, where low temperature heat is required. In this combination the PVT collector improves efficiency in two ways-of the heat pump by additional low temperature heat and of the photovoltaic module by lower cell temperatures. An increase of 4% of the annual and up to 9% for the daily photovoltaic electricity production and a high collector yield of 450 kWh/(m² a) was measured at a pilot system between April 2009 and April 2010. As the improvement of the PV and heat pump efficiency depends strongly on the thermal PVT collector performance, efficiency measurements of five different unglazed PVT collectors have been carried out. They showed significant differences, e.g. the conversion factor 0 without electricity production varies from 0.33 to 0.73. Further, a simulation model for unglazed PVT-collectors is presented. It is based on thermal performance parameters of EN 12975 and PV performance data at standard test conditions and it further includes thermal collector capacity and condensation effects.

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Evaporation in solar thermal collectors normally takes place when the collector pump is not running—the so-called full stagnation. But it is possible that part of the heat transfer fluid evaporates inside a solar thermal collector field although the pump is operating and the collector field outlet temperature is significantly below the evaporation temperature. This operating status is called partial stagnation since only parts of the collector are affected by evaporation. Partial stagnation happens at a pronounced nonuniform temperature distribution in combination with a low mass flow rate and/or a high temperature level. A main reason for an irregular temperature distribution is a nonuniform flow distribution inside the solar thermal system. The paper presents an experimental investigation that analyzes the reasons and effects of partial stagnation occurrences. For this, outdoor measurements were made with a direct-flow vacuum tube collector. Criteria that promote partial stagnation have been identified, such as a coaxial tube design, a low system pressure, and a high gas content of the fluid. Performance measurements show no efficiency reduction during partial stagnation in the system investigated at a horizontal or positive collector slope. A high degree of partial stagnation, however, might pass into a complete evaporation of the collector volume although the collector pump is still running. This could lead to a complete blockage of the flow and a high thermal load of the system components. In all cases, partial stagnation leads to an unstable operation and a high load of the collector fluid and should, therefore, be avoided by design measures. A minimized risk for evaporation during operation is achieved by a more equal flow distribution inside the collector and the whole collector field, air bubbles, and solid particles should be completely removed. In addition, the gas content dissolved in the fluid may be reduced and the system pressure level may be increased in order to raise the boiling temperature.

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