Energy Conversion and Management: X (Oct 2023)
Exergy-Optimum coupling of radiant panels with heat pumps for minimum CO2 emission responsibility
Abstract
This paper introduces an exergy-based optimization model for radiant heating or cooling panels coupled with heat pumps for minimum carbon dioxide emissions responsibility. The general idea that there exists an optimum tube spacing in radiant panels for minimum carbon dioxide emission responsibility in terms of simple life-cycle analysis and the first law of thermodynamics is challenged. Using the Rational Exergy Management Model, it is shown that a real optimum is unavailable according to the second law with or without using heat pumps or boilers for temperature peaking or chillers for cooling. It is hypothesized that the use of temperature-adjusting heat pumps is not an exergy-optimum choice. Through twelve sets of optimization constraints, identified in this paper, it is shown that potential optimality conditions concerning the increase of emission embodiments of the radiant panel material and labor with respect to narrover tube spacing versus an increase in the coefficient of performance of the heat pump, thus a decrease in operational emissions, lies outside the feasible design range. This paper also reminds that heat pumps destroy part of the high unit exergy of the electrical power demand by generating only low-exergy thermal energy. It is further shown that delegating solar photo-voltaic panels for thermal and electrical power supply as well as temperature peaking provides optimal solutions both for radiant panel heating and cooling, enabling wide use of low-enthalpy energy sources. Three case studies are presented. Case 1: Hydronic floor heating with temperature-peaking heat pump. This case has the highest emissions responsibility among other cases. It is + 0.095 kg CO2/kW-hexergy of heat delivered per unit panel area. Case 2: Oversized radiant floor heating with closer tube spacing without a heat pump. This case is responsible for + 0.035 kg CO2/kW-hexergy of heat delivered per unit panel area. 3-a: Heat-piped solar photo-voltaic-thermal panels with floor heating with heat pipes. This case has a negative emissions responsibility of −0.010 kg CO2/kW-hexergy of heat delivered. 3-b: This case is the radiant ceiling cooling option of Case 3-a. It is responsible for + 0.0010 kg CO2/kW-hexergy of sensible cooling. Results show that negative-carbon solar heating with radiant panels may be achieved with real roots available for optimum tube spacing. Optimal tube spacings for ceiling cooling panels also exist.
