Ground source heat pump (GSHP) systems are globally recognized as one of the most promising technologies in terms of economic and energy savings. However, despite the aroused interest, operative performances can be lower than expected, possibly reducing the attractiveness of GSHPs with respect to other solutions (e.g. air heat pumps, condensing boilers, solar technologies). The reasons can be ascribed to an incomplete technological development, together with not-optimized sizing and control strategies. A comprehensive model was introduced in previous works in order to develop and test innovative sizing and control methodologies [1]. It was shown that traditional design approaches tend to oversize heat pump capacity and number of boreholes, resulting in lower operative performances and higher installation costs. It was also illustrated how an optimized synergy between geothermal source and back-up generators leads to remarkable energetic and economic savings. Together with advanced design and control methodologies, additional technological developments may be necessary to improve the current performance of GSHPs. As well known, GSHP systems involve different subsystems: ground source, ground-coupled heat exchangers, heat pump unit, and back-up generators; the present work aims at identifying their relative influence on the overall efficiency of the system and the limits to which technological improvements have to be pushed (because, beyond these limits, the achievable benefits start to be negligible). To this end, an analysis of thermodynamic losses is conducted for a case study, followed by a sensitivity analysis on the heat pump unit thermal performance. Primary energy consumptions of nine configurations with different combinations of ideal and real subsystems are compared. The completely ideal system is used as the reference to normalize energy consumptions and obtain a dimensionless efficiency parameter, always lower than the unity. The results show that – when a proper design methodology is employed – the performance of the BHEs slightly affects the overall efficiency. On the contrary, the thermal response of the ground and the thermal and hydraulic performances of the heat pump unit are key factors. This notwithstanding, the sensitivity analysis, conducted by increasing the efficiency of the heat pump unit, shows a saturation trend of the system performance, hinting that large technological improvements are not properly “rewarded”, unless higher thermal capacity and conductivity of the ground are available. Besides, different behaviors in heating and cooling modes suggest a preferable path for technological development.
Analysis of Thermodynamic Losses in Ground Source Heat Pumps and Their Influence on Overall System Performance
CASAROSA, CLAUDIO;CONTI, PAOLO;FRANCO, ALESSANDRO;GRASSI, WALTER;TESTI, DANIELE
2014-01-01
Abstract
Ground source heat pump (GSHP) systems are globally recognized as one of the most promising technologies in terms of economic and energy savings. However, despite the aroused interest, operative performances can be lower than expected, possibly reducing the attractiveness of GSHPs with respect to other solutions (e.g. air heat pumps, condensing boilers, solar technologies). The reasons can be ascribed to an incomplete technological development, together with not-optimized sizing and control strategies. A comprehensive model was introduced in previous works in order to develop and test innovative sizing and control methodologies [1]. It was shown that traditional design approaches tend to oversize heat pump capacity and number of boreholes, resulting in lower operative performances and higher installation costs. It was also illustrated how an optimized synergy between geothermal source and back-up generators leads to remarkable energetic and economic savings. Together with advanced design and control methodologies, additional technological developments may be necessary to improve the current performance of GSHPs. As well known, GSHP systems involve different subsystems: ground source, ground-coupled heat exchangers, heat pump unit, and back-up generators; the present work aims at identifying their relative influence on the overall efficiency of the system and the limits to which technological improvements have to be pushed (because, beyond these limits, the achievable benefits start to be negligible). To this end, an analysis of thermodynamic losses is conducted for a case study, followed by a sensitivity analysis on the heat pump unit thermal performance. Primary energy consumptions of nine configurations with different combinations of ideal and real subsystems are compared. The completely ideal system is used as the reference to normalize energy consumptions and obtain a dimensionless efficiency parameter, always lower than the unity. The results show that – when a proper design methodology is employed – the performance of the BHEs slightly affects the overall efficiency. On the contrary, the thermal response of the ground and the thermal and hydraulic performances of the heat pump unit are key factors. This notwithstanding, the sensitivity analysis, conducted by increasing the efficiency of the heat pump unit, shows a saturation trend of the system performance, hinting that large technological improvements are not properly “rewarded”, unless higher thermal capacity and conductivity of the ground are available. Besides, different behaviors in heating and cooling modes suggest a preferable path for technological development.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
