Comparison of Proportional and On/Off Solar Collector Loop Control Strategies Using a Dynamic Collector Model

Abstract

Common control strategies used to regulate the flow of liquid through flat-plate solar collectors are discussed and evaluated using a dynamic collector model. Performance of all strategies is compared using different set points, flow rates, insolation levels and patterns, and ambient temperature conditions. The unique characteristic of the dynamic collector model is that it includes the effect of collector capacitance. Short term temperature response and the energy-storage capability of collector capacitance are shown to play significant roles in comparing on/off and proportional controllers. Inclusion of these effects has produced considerably more realistic simulations than any generated by steady-state models. Simulations indicate relative advantages and disadvantages of both types of controllers, conditions under which each performs better, and the importance of pump cycling and controller set points on total energy collection.

On Differential Temperature Controller Setpoint Selection for Active Photovoltaic-Thermal (PV-T) Systems

“…(1) -since compliance is generally handled conservatively in light of model limitations, parameter uncertainty, the limited thermal performance sensitivity to low Δ setpoints (≈ 1 K), and temperature measurement tolerances (1-3 K) (Winn, 1983;Hirsch, 1985;Peuser et al, 2002). On the other hand, the condition for cycling-free operation is generally regarded as merely indicative, if not unreliable, hence taking the effect of PV conversion into consideration would be of little practical consequence (Duffie and Beckman, 2013;Schiller et al, 1980;Winn, 1983).…”

“…Previous research efforts have largely determined how and to what extent controller setpoints can be expected to influence the thermal energy collection process of non-hybrid ST systems. In essence, higher ΔTon and ΔToff setpoints tend to reduce pump use by causing fluid circulation to start later and stop earlier, respectively, than they would otherwise, which tends to negatively influence the thermal energy collected although not necessarily in a monotonic way, while small deadbands (ΔTon -ΔToff) tend to exacerbate pump cycling (Schiller et al, 1980;Winn, 1983;Hirsch, 1985;Kahwaji and Winn, 1986;Muralidhar et al, 1989). Magnitude-wise, modest ΔTon setpoint increases (5-10 K) relative to low initial setpoints (2-5 K) have been estimated to decrease the daily thermal energy collection efficiency by as much as 26% (absolute), though most severely for low yield days, while shortening pump running times by 3-69% (relative) (Schiller et al, 1980;Kahwaji and Winn;1986;Huang, 1994).…”

“…In essence, higher ΔTon and ΔToff setpoints tend to reduce pump use by causing fluid circulation to start later and stop earlier, respectively, than they would otherwise, which tends to negatively influence the thermal energy collected although not necessarily in a monotonic way, while small deadbands (ΔTon -ΔToff) tend to exacerbate pump cycling (Schiller et al, 1980;Winn, 1983;Hirsch, 1985;Kahwaji and Winn, 1986;Muralidhar et al, 1989). Magnitude-wise, modest ΔTon setpoint increases (5-10 K) relative to low initial setpoints (2-5 K) have been estimated to decrease the daily thermal energy collection efficiency by as much as 26% (absolute), though most severely for low yield days, while shortening pump running times by 3-69% (relative) (Schiller et al, 1980;Kahwaji and Winn;1986;Huang, 1994). In turn, Hirsch (1985) argued the thermal performance sensitivity to ΔToff was low for typical setpoints around 1 K while Muralidhar et al (1989) summarised experiments suggesting a higher sensitivity to ΔToff than to ΔTon.…”

“…These design rules can provide useful information for the purposes of setpoint selection. Equation (1) provides sound estimates for the minimum Δ setpoint guaranteeing cost-effective pump operation in the presence of measurement errors, and regardless of whether or not timers are used, and thus has strong practical relevance (Schiller et al, 1980;Kahwaji and Winn, 1986;Winn, 1993;Alcone and Herman, 1981). Conversely, Eq.…”

“…The first holds that maintaining a given ratio between Δ and Δ is a condition for stable operation in active ST systems, whereas the second consists of largely undifferentiated setpoint ranges. Schiller et al (1979) claim the ratios typically vary from 2 up to 7, whereas Winn (1993) cites ratios between 4 up 6 for Δ setpoints between 1 and 2 K, and highest when Δ is lowest. In turn, the setpoints reported in the literature fall into the following ranges: 3-15 K, for Δ ; and, 0.2-6 K, for Δ (Kalogirou, 2009;Winn, 1993;Beckman et al, 1994;Prapas et al, 1995;Knudsen, 2002;Eicker, 2003;Streicher and Kaltschmitt, 2007;Badescu, 2008).…”