Impact of Solar Estimation on MPC Performance of Multi-zone Buildings

Transcription

1 Impact of Solar Estimation on MPC Performance of Multi-zone Buildings Donghun Kim 1 *, Lucas Witmer 2 *, Jerey R. S. Brownson 2, James E. Braun /07/14 1 Ray W. Herrick Laboratories, School of Mechanical Engineering, Purdue University 2 John and Willie Leone Family Department of Energy and Mineral Engineering, Pennsylvania State University

2 MOTIVATION AND PURPOSE Model Predictive Controllers in building control may employ solar irradiance data to make decisions Solar irradiations on surfaces are calculated by series of estimates and empirical correlations based on Global Horizontal Irradiance (GHI). Control decisions based on the solar model are likely wrong due to the imperfect information. Investigate the impact of uncertain solar estimation on MPC performance w.r.t. energy consumption and comfort. We will compare MPC responses for a multi-zone building based on: Measured local POA solar information Modeled POA solar information 2

3 SUMMARY OF CASE STUDY BUILDING Building 101, Navy yard Philadelphia Zone Temp. SP One AHU serves 9 zones T z, T sup will be determined Compressor power >> Fan Power Supply air temp. SP 3

4 MEASURED VS MODELED TOTAL SOLAR RAD. June/01/2012 July/31/2012 Modeled solar was calculated by solar model, Perez model with shaded Filter model, based on GHI. Somewhat unbiased 4

5 MPC ASSUMPTIONS FOR ANALYSIS Assumptions for analysis MPC contains a perfect model for HVAC/Envelope system Perfect forecaster for gain/weather Local controller perfectly track set-points decided by MPC unless a saturation occurs. Perfect state estimation to run MPC controller MPC Objective N p min RPT ( ( k), T ( k), Q ( k), wk ( )) t st.. k 1 k z, sp sup, sp load BT u z, sp Bww k Cx k k w k x k 1 Ax k k k Q D T D f(p, T, T, Q,w) 0 load u z, sp w z, sp sup, sp load Tsp Tsp ( k) Tsp 1 Stage( k ) m ( k) 1 VAV, j 5

6 SCHEMATIC FOR COMPARISON Gains except solar Measured solar Modeled solar reference HVAC/Envelope Model modeled MPC controller T sp T sup,sp State feedback rmpc (red): MPC response with measured solar mmpc (blue): MPC response with modeled solar 6

7 CASE STUDY BUILDING External view of zone 5, east side view External view of zone 6, east side view Particularly interested in zone 5(E) and 6(W). Around 20% window area ratio 7

8 OPTIMALITY CONDITION OF MPC FOR MULTI-ZONE Optimality condition : At least one VAV damper should be fully opened to be optimal for the system. (proof by contradiction). critical zone:= a zone with fully-opened damper 8

9 A CASE 0 AND ISSUES Energy [kwh/day] Peak Power [kw] mmpc rmpc Interestingly, no significant difference w.r.t. energy and comfort Relative energy consumption difference is less than 0.1 % Comparisons of rmpc (red solid) and mmpc (blue dahsed) for Case 0 (July 8, 2013) 9

10 REASON FOR THE SMALL EFFECT The load of ground floor is high and it has an undersized VAV box. Due to the undersized VAV, the zone requires low T sup,sp to meet the high load. Other zones have potential capacity to meet their load perturbations only by adjusting VAV dampers no comfort issue no energy issue since compressor power >> fan power Adjustments for internal gain for each zone were necessary to investigate the solar impact. 10

11 CASE 1 Solar radiations are overestimated. mmpc decides lower T sup,sp assuming that one of z5 & z6 is fully opened. However actual load is smaller than the expected. Local controllers require less air flow rates to track T sp than MPC expected due to overcooled T sup. mmpc takes more energy, around 4.6%. No comfort violation. Case1: Overestimated POA Solar (July 8,2013) 11

12 CASE 2 mmpc decides higher T sup,sp assuming that one of z5 & z6 is fully opened. However actual load is higher than the expected. No option for local controllers to track T sp with undercooled T sup. Comfort violation occurs, peak 2.69 o C Takes less energy, 2.68%. Case2: Under estimated POA Solar (July 8,2013) 12

13 CASE 3 Same explanation holds. Overestimation energy cost, No comfort cost Underestimation energy cost, comfort cost Case3: Under /Overestimated POA Solar (July 8,2013) 13

14 CASE 4 Energy saving potential Affects on degrees of precooling significantly Energy: +0.43% Peak comfort violation: ~ 3 o C Case4: 5 hour MPC 14

15 MONTHLY SIMULATION WITH 5HR MPC Energy consumption [kwh/day] Peak comfort violation [ o C] Conv mmpc rmpc MPCs saves around 12 (%). The level of energy saving are similar. Comfort issue occurs for mmpc. Significant comfort violation: Peak:~3 o C Duration: ~4 hr 15

16 CONCLUSIONS When a critical zone is insensitive to solar, When a critical zone is sensitive to solar, the impact on energy consumption is negligible due to the unbiased solar estimation. comfort violation occurs when underestimation case The degree of comfort violation is dependent on not only the building envelope, e.g. window area ratio, but also is dependent on the HVAC equipment, e.g. size of the VAV, and internal gain profiles. For high percentage window area buildings (DOE BTO Commercial Reference Medium Office~60%, cf Building 101~20%), the comfort issue will be more significant. To prevent the situation, pressure reset strategy needs be considered with MPC. Although this study focuses on solar, similar responses are expected for load estimation/prediction error. 16