Saving Energy using PICVs

Transcription

1 Saving Energy using PICVs Hw Dynamic-Balancing f Hydrnic Systems Yields up t 30% in Energy Distributin Savings Keeping temperatures under cntrl PICVs cmbine cst savings and cmfrt Pressure-independent cmbi valves (PICVs) play an imprtant rle in reducing energy cnsumptin while maintaining building temperature at ptimal setpints. PICVs are effective because they use dynamic-balancing t handle pressure fluctuatins in a building s hydrnic system. Dynamic-balancing has tw majr functins. First, it prevents the versupply f cnsumers and the subsequent hydrnic interference. Secnd, it drastically reduces temperature swings. As a result, the system uses less energy t maintain ccupant cmfrt. In additin, PICVs have a pre-setting functin that prvides even finer temperature cntrl accuracy, further eliminating temperature fluctuatins and discmfrt. As a result, ccupants are less likely t raise r lwer temperature settings, adding t the verall energy savings that the valves generate. PICVs als allw fr advanced pump cntrl strategies that reduce energy use even mre. In ttal, PICVs can generate energy savings f up t 30%. They can be used in almst any heating and cling applicatin t prvide year-rund cmfrt fr building ccupants. This paper discusses the energy savings methds in detail and includes a case study example that quantifies them. Siemens Switzerland Ltd, 2017 siemens.cm/acvatix1

2 PICVs in the Hydrnic Cntext Dynamic-Balancing Against Pressure Differences Pressure-independent cmbi valves (PICVs) ensure that the flw f ht r cld water is slely dependent n valve travel. Within their range f peratin, they are nt affected by pressure fluctuatins in the building s hydrnic system. This is called dynamic-balancing r aut-balancing. This basic functinality is achieved by an internal differential pressure regulatr (Figure 1, #3) wrking in series t the main flw cntrl valve (#1) and regulating the pressure differential f the flw cntrl valve using a pressure inlet and membrane. Hence the flw acrss the entire device is independent f the pressure changes in the system and is determined nly by the travel f the cntrl valve. PICVs prvide the same actuatr interface as standard cntrl valves. It isn t necessary t have additinal external energy supply r an electrical sensr. The energy t perate the differential pressure cntrller is prvided by the hydrnic system itself. Anther cre functin f PICVs is t limit the maximum desired flw. Typically this is dne either by limiting the flw cntrl valve s travel r by limiting the free cntrl path area (#2). Enabling Energy Savings in Three Different Ways In heating and cling applicatins in a building, the autbalancing functin generates energy savings in three different ways: It eliminates heat exchanger verflw at anytime and under any perating cnditin. It imprves cntrl accuracy by eliminating hydraulic crss-cupling between neighbring cntrl lps. It enables advanced energy distributin strategies by eliminating the risk f heat exchanger starvatin PICVs Are Relevant in the Whle Hydrnic System PICVs can be used in almst all heating and cling applicatins in a building, including energy generatin, distributin and cnsumptin. The mst typical are: Energy Cnsumptin Chilled ceilings Radiatrs Heating/chilled water zne cntrl Heating/cling cils in: - Fan cil units - Air handling units - VAV systems (Variable Air Vlume) Energy Distributin Heating grup Chilled water grup Energy Generatin District heating Figure 1: Schematics f a mechanical PICV 1. Flw cntrl valve 2. Pre-setting 3. Differential pressure regulatr Siemens Switzerland Ltd,

3 Aviding Overflw Different Resistance Leads t Under- r Oversupply In hydrnic heating and cling systems, the ht r cld medium distributing the thermal energy frm its generatin t the cnsumer (water, either plain r mixed with an agent like glycl) is transprted ver piping sectins f different lengths and diameters. In the case f multi-stry buildings, the elevatin t vercme may als vary. As a cnsequence, the hydraulic resistance alng the path frm the energy generatr t each terminal unit is different. T prvide the required heating r cling, each terminal unit is designed fr a certain flw. When the flw is t lw, the cnsumer des nt receive enugh energy (undersupply). In the ppsite case, when an verflw (r versupply) takes place, the flw is s high that the terminal unit cannt sufficiently exchange the thermal energy prvided. As a cnsequence, the excess energy is sent back t the energy generatr, which is then unable t perate at peak efficiency. The Differences Are Nrmalized with Static Balancing In rder t ensure that every cnsumer receives the prper amunt f heating/cling energy, hydraulic resistance is intrduced int the system. Cnventinally, this s-called balancing is dne by installing manual balancing valves (MBVs), which are installed in series t the standard regulatr valves. In this methd, the hydraulic resistance f the MBVs is dimensined s that the system is perfectly balanced fr nminal perating cnditin. The system is statically balanced. Hwever, this can nly be achieved fr ne given ideal perating cnditin (Fig. 2). Fr example, if sme f the circuits are nly half pen (partlad cnditin) and the rest are fully pen (full-lad cnditin), an verflw takes place in the latter circuits, which get excessive energy (Figure 3). Figure 3: When certain circuits are in part-lad r clsed, thers are in verflw (large blue arrws). An verflw might last fr quite sme time befre the rm temperature cntrller reacts t the increased r decreased temperature. Such transient verflw phases usually ccur either due t a change f lad (e.g., change f ccupancy f a rm) r due t a change f setpint (e.g., start-up phase in the mrning). Overflw Leads t Energy Inefficiencies Depending n the type f energy generatrs, this verflw may lead t tw negative side effects. First, verflw leads t the transprtatin f water thrugh the system that desn t carry a prper amunt f additinal energy t the cnsumers, 1 and hence a lw temperature difference acrss the heat exchanger. Secnd, in the case f chillers and heat pumps, verflw causes inefficiencies in the energy generatrs. Overflw f dedicated cnsumers can lead t a return temperature lwer than the nminal design value in cling mde and a return temperature higher than the nminal design value in heating mde, decreasing the energy efficiency f bilers and chillers by 2% and 3%, respectively. 2 Figure 2: Statically balanced system perating at the design perating pint. Overflw Still Happens in Spite f Static Balancing The reality lks quite different, hwever. In statically balanced systems, verflw may still ccur in certain partlad cnditins. 1 Heat transfer by the heat exchanger is directly prprtinal t the flw rate and the temperature difference acrss the heat exchanger. Flw rate and temperature difference are inversely prprtinal t each ther in a clsed system. 2 A decrease f the evapratin temperature f a chiller belw its design value by 1 degree decreases its perfrmance by arund 3%. Increasing the cndensing temperature f a heat pump ver its design value by 1 degree decreases its perfrmance by arund 2%. Siemens Switzerland Ltd,

4 Imprving Cntrl Accuracy PICVs Eliminate Overflws with Dynamic-Balancing As stated in the descriptin f the PICV wrking principle, the use f PICVs limits the maximum flw at part-lad cnditins and thus avids the mentined increase f direct energy demand (generatin, cnsumptin) and indirect energy demand (transprt, distributin). Hydraulic Crss-Cupling Triggers Variatins f Temperature in the Building As described abve, a sectin f the heating r cling system may temprarily increase (r decrease) its energy demand, fr instance when a meeting rm is filled with peple at the beginning f a wrkshp r empties at the end. This happens everywhere in the building, at different mments, in different places. This increase in energy demand in certain sectins f the system leads t a reductin f the energy supplied t ther areas f the building. The temperature f these areas then deviates frm the setpint and it takes time until the rm thermstat triggers the apprpriate respnse. The temperature will then fllw a cycle f increases and decreases f temperature and stabilize again ver time arund the desired setpint (Figure 4). This effect is called hydraulic crss-cupling. The first issue with hydraulic crss-cupling is that users f the building experience perids f discmfrt when the temperature is at its lwest r highest pint in the cycle. Users Shift the Setpint t Reduce Discmfrt The secnd issue is that users will typically change the temperature setpint when they experience sme degree f discmfrt. Fr example, when the temperature is at its lwest pint in the cycle during the cld mnths, they may increase the setpint by a cuple f ntches. The whle curve is shifted up ne r tw degrees. Hwever, they will likely nt react an hur later, when the rm temperature is a bit higher than usual. The setpint shift stays fr the whle seasn. A similar scenari takes place during the ht mnths. When the rm is at the httest temperature in the cycle, the users may crank up the cling, withut turning it back dwn later n when the temperature is at the lwest pint. In bth heating and cling cases, the verall energy demand is increased because f the variatins in temperature caused by the hydraulic disturbances. PICVs Nearly Eliminate Temperature Variatins When PICVs are used, their aut-balancing functinality cmpensates fr the variatins in pressure. They allw fr much better cntrl accuracy at the setpint and hence virutally eliminate the temperature swings (Figure 5). Figure 4: Because f crss-cupling, the temperature deviates frm the setpint. Delayed crrectin f the rm temperature leads t wide temperature fluctuatin, less cmfrt and energy lss. Figure 5: PICVs autmatically cmpensate fr the variatins in pressure and maintain the rm temperature very clse t the setpint. Siemens Switzerland Ltd,

5 Full-Strke Further Increases Cntrl Accuracy Even finer cntrl accuracy is prvided by Siemens PICVs, in which pre-setting is btained by limiting the free cntrl path area. Since the full travel f the flw cntrl valve is available t manage the pening, the vlumetric flw can be defined using a much larger number f steps (Figure 6). The temperature can be reached in smaller increments, thus reducing even further the temperature fluctuatins and discmfrt. Preventing the Setpint Shift Leads t Energy Savings As a cnsequence, users d nt experience any discmfrt at the riginal setpint and will nt shift energy demand up t cmpensate fr the peaks f temperature variatins. When carried ver the entire building fr an entire seasn, this adds up t substantial energy savings Figure 6: PICVs with pre-setting btained by limiting the flw cntrl valve s travel (strke limitatin) have reduced cntrl accuracy (purple). Siemens PICVs with pre-setting btained by limiting the free cntrl path area still have the full strke available and prvide a much mre granular cntrl f the flw and temperature (green). Enabling Optimal Distributin Strategies Cnventinal Systems Require Cnstant Pressure Mdern energy transprtatin systems, such as variablespeed cntrlled pumps, adapt the delivery head f a pump and the vlume flw t the demand lad. There are a variety f cntrl strategies n the market tday. Cntrl can be dne in cnnectin with differential pressure, effective vlume flw thrugh a flw sensr, differential temperature, utside temperature, r supply temperature. As explained abve, a cnventinal hydrnic system is statically balanced. The hydraulic resistance f the MBVs is dimensined s that the system is perfectly balanced fr a nminal perating cnditin. Typically, since such a system might still be sensitive t pressure differences, the pump cntrl strategy is designed t ensure a cnstant differential pressure in the system (Figure 7). Pumps Have t Battle against Unnecessary Resistance Any reductin f the pressure difference culd lead t the starvatin f sme terminal units. Even when fully pened, they d nt get the necessary flw required. As a cnsequence, the energy exchange is insufficient and the temperature setpint cannt be ensured anymre. T ensure the necessary flw, pumps have t perate against the hydraulic resistance that has been intrduced int the system t ensure a nminal perating cnditin, even if the actual perating cnditins are much different. Figure 7: Pump cntrl strategy ensuring that differential pressure is cnstantly maintained at the desired value. Siemens Switzerland Ltd,

6 As PICVs Maintain Flw, Pumps Optimize Pressure On the ther hand, PICVs make it pssible t deliver the same flw at a lwer pressure difference. As lng as the pressure difference remains in the allwed PICV perating range, the flw will be maintained at the set level (autbalancing feature). This pens the dr fr advanced pump cntrl strategies, where the same flw is delivered at a lwer pressure difference, smewhere between the lwest pssible pint (t remain in the PICV s perating range) and the nminal pint (Figure 8). The pump battles against less resistance. It can perate at an ptimal speed and requires substantially less energy t prvide the same perfrmance. Figure 8: Pump cntrl strategy with variatin f differential pressure. Savings frm a Real Case Applicatin in Real Life Case Study The three ways t generate savings described in this paper were implemented in a campus with several buildings in a large Saudi Arabian city that has a representative number f heating and cling days. This building features air handling and fan cil units, with chilled water fr cling and electrical re-heaters fr heating. The chilled water system incrprates the fllwing cmpnents: 10 chillers. Lcated at utility building. Nine duty and ne standby, capacity: 1370 kw each. 10 primary chilled water pumps, cnstant speed. Lcated at utility building. Nine duty and ne standby, capacity: 55 l/s (198 30m head. The rati f installed pump capacity and installed cling capacity (chillers) is apprximately 1.5%. 10 secndary chilled water pumps, variable speed. Lcated at utility building. Nine duty and ne standby, capacity: 55 l/s (198 55m head. The rati f installed pump capacity and installed cling capacity (chillers) is apprximately 2.5%. Different sizes f air handling units (AHUs) and fan cil units (FCUs) lcated at each building as per demand cling lads. Cntrl valves with electrical actuatrs installed n the chilled water return pipes f the cling units (AHUs and FCUs). Up t 30% Savings with PICVs Using actual perating and climatic data, energy savings were generated fr bth energy distributin and energy generatin using the fllwing three methds: Eliminating heat exchanger verflw at any time and under any perating cnditin Imprving cntrl accuracy by eliminating hydraulic crss-cupling between neighbring cntrl lps Enabling advanced energy distributin strategies by eliminating the risk f heat exchanger / cling cils starvatin In this case, cnservative calculatins demnstrated that using PICVs in the building yielded savings f up t 25-30% in energy distributin and savings f 2-5% fr energy generatin. In abslute annual figures, these savings amunted t apprximately 330 MWh and apprximately 200 MWh, respectively, r a ttal annual cst saving f arund EUR. Siemens Switzerland Ltd,

7 Published by Siemens Switzerland Ltd 2017 Building Technlgies Divisin Internatinal Headquarters Gubelstrasse Zug Switzerland Tel Siemens Switzerland Ltd,