Chapter 18B: THERMAL ENERGY STORAGE Agami Reddy (July 2016) - Diurnal and seasonal variability in load - Elements of utility rate structure - Some examples of rate structures - Benefits of TES - Design sizing options - full storage - partial storage- load leveling - partial storage- demand limiting - Different types of ice storage - Solved example - Series and parallel system configurations - Effectiveness- NTU method for TES performance HCB 3- Chap 18B: TES

Loads vary greatly- diurnally and seasonally (actual loads for an electric utility) HCB 3- Chap 18B: TES

Load Variability The electric loads which an utility must meet varies appreciably from hour to hour and from day to day The PUCC requires that the electric utility have enough generating capacity to meet peak loads or have power purchase agreements with other utilities Peak power much more expensive than base load Certain types of power plants (such as coal or nuclear) take several hours to bring a turbine on-line. Others (such as gas-turbines) can be brought online quickly. HCB 3- Chap 18B: TES

Diurnal Variability Load factor = average energy use / peak demand HCB 3- Chap 18B: TES

Typical Load Curve for a Large 10/20/2017 Typical Load Curve for a Large Urban Building Load Curve for a Typical Urban Utility Load factor = average energy use / peak demand HCB 3- Chap 18B: TES Revised version 5. Jay Golden ASU

Electric Rate Structure- Cost Components for Utility Customer associated costs: arising from connection between customer and utility (original metering, maintenance of service…) Energy/commodity costs: to meet actual energy used by customers (electricity and gas) Demand costs: this is intended to compensate the utility for: (i) the stranded cost of excess generation and distribution capacity, or (ii) to purchase electricity from other utilities at premium rates (In order to meet the peak power as mandated by the Public Utility Commission-PUC) HCB 3- Chap 18B: TES

Elements of Utility Rate Structure Customer charge: flat fee per customer Energy charge: - charge for use of energy (kWh or ft^3) - fuel adjustment factor (allows utility to change the price allocated for fuel recovery on a monthly/quarterly/annual basis without going to PUC for formal approval) Demand charge based on peak or max use - usually not applied to residences or small commercial - based on a demand meter which registers the maximum use during a specified time window (usually 15 min or 30 min or hourly) HCB 3- Chap 18B: TES

Peak Sifting has large benefit to electric utility MacCraken, Dec. 1991 HCB 3- Chap 18B: TES

Two of the numerous types of rate structures 10/20/2017 Two of the numerous types of rate structures Typical Three Tier Time-of-Use (TOU) Rate Typical Demand Charge for a Large Office Building (Ratchet) HCB 3- Chap 18B: TES Revised version 5. Jay Golden ASU

Typical Usage Patterns What is average kW? From chap 18- Energy Management Handbook by Turner and Doty, 6th Ed. HCB 3- Chap 18B: TES

Example 1- Using Utility Bills HCB 3- Chap 18B: TES

APS Electric Utility HCB 3- Chap 18B: TES

Benefits of TES: Primarily Operating Cost Savings Electric cost savings from load shifting Peak demand reduction Favorable TOU charge differentials Increased efficiency? Site Source and transmission HCB 3- Chap 18B: TES

Why use Thermal Storage? For customers Lower energy bills for a facility billed separately for on-peak and off-peak electricity rates Lower demand charges: reduces electrical peaks during day time hours so as to minimize electric demand Cooling equipment can be downsized Storage can replace a chiller in multi-chiller facilities Provides emergency/critical cooling capability Very energy efficient for lower air temperature HVAC applications. For utilities Reduces peak demand and fills load valleys Improves load factor: better utilization of baseload generating equipment and higher off-peak sales Reduces reliance on peaking units Allows defering capacity expansion HCB 3- Chap 18B: TES

Design / Sizing Options Depends on how TES is to be operated Design operating cycle-usually 24 hours Longer cycle, typically 7 days, used to take advantage of lower average loads and charge TES during weekend How much storage (energy)? Maximum charge/discharge rates (power)? When to discharge? Determines sizing method Utility peak period As needed to meet load Demand limit HCB 3- Chap 18B: TES

Option 1: Full Storage (Daily) Shift ENTIRE on-peak cooling load during a high utility cost period to off-peak periods Typically, cooling system operates at full capacity during all non-peak hours on design day This strategy results in “Largest” peak demand shift storage requirement refrigeration requirement Driven by operating cost savings Rarely the best choice w/o external drivers Simple control of system HCB 3- Chap 18B: TES

Option 2: Partial Storage- Load Leveling Shift part of load from on-peak to off-peak Several strategies possible Special case—load leveling Plant operates at full capacity (load factor = 100%) on design day for all 24 hours Minimum refrig capacity requirement Moderate storage requirement Chiller capacity ~total load/cycle length Smaller load shift than full storage option Near optimal payback Option particularly attractive for applications where the peak cooling load is much higher than the average load HCB 3- Chap 18B: TES

Option 3: Partial Storage-Demand Limiting This is a middle ground strategy between load shifting and load leveling Cooling equipment operates at a reduced capacity or reduced demand level during on-peak period Use storage discharge to control aggregate demand, i.e., control cooling equipment to keep facility billed demand below a certain level High peak discharge rate relative to amount of storage Must predict electric demand profile over day Demand savings as well as equipment costs are higher than load-leveling strategy and lower than load-shifting strategy Monitoring devices required for proper operation More analysis effort involved but more Commonly used design method HCB 3- Chap 18B: TES

Ice-On-Coil Ice formed and stored on tube heat exchanger surface Coolant (glycol or DX) on tube side Modular tanks (100-200 Ton-hrs) Ice-On-Coil HCB 3- Chap 18B: TES

Ice on coil storage configurations Internal melt External melt HCB 3- Chap 18B: TES

FIGURE 18.33 Different stages of charging an ice on coil system Somewhat complex to model due to constrained growth FIGURE 18.33 Different stages of charging an ice on coil system (a) First stage where ice has yet to be formed, (b) Second stage of charging where ice forms around the coils and grows in an unconstrained manner, and (c) Constrained growth where the ice layer formed around adjacent tubes touch one another and only the water in-between the tubes is gradually frozen HCB 3- Chap 18B: TES

Outdoor Application HCB 3- Chap 18B: TES

Chilled Water Storage Typical stratification temperature profile Naturally stratified chilled water storage tank with diffusers and typical stratification temperature profile HCB 3- Chap 18B: TES

Example : Idealized comparison of different CTES sizing options   Building whose cooling load in tons is shown in 2nd column of Figure 18.30 on next slide. Assume no heat losses from the piping and the CTES tank, and that the figure of merit of the storage tank is 100%. The on-peak period: from 11:00 to 18:00 hour ending (i.e., for 8 hours). The chiller sizing and hourly cooling output as well as the state of charge of the CTES under different design options are summarized on an hour-by-hour basis on next slide. HCB 3- Chap 18B: TES

Figure 18.30 HCB 3- Chap 18B: TES

HCB 3- Chap 18B: TES

Series Flow- Chiller Priority Chiller meets as much of the load as possible Three-way control valve allows flow through the CTES to provide supplemental cooling in case load exceeds the capacity of the chiller. Since the chiller sees warm fluid returning from the building load, its operation would be more efficient (higher COP). On the other hand, the usable portion of the total nominal storage capacity is reduced because of the lower storage temperature. This strategy is used in instances where the cost of stored energy is higher than the cost of direct cooling or when the utility savings through on-peak demand reduction is high. The control is simple to implement and is thus commonly used in most chilled water systems and in ice storage systems as well. HCB 3- Chap 18B: TES

Series Flow- Storage Priority - System configuration is meant to deplete as much of the cooling from storage as possible prior to requiring supplemental assistance from the chiller. - While the chillers would tend to operate less efficiently, a higher usable storage capacity can be achieved. - However, the control sequence tends to be more complex. HCB 3- Chap 18B: TES

Parallel Flow Configuration - There are several variants of the parallel flow configuration, one of which is shown - This system is more suitable for retrofit applications that require a fixed temperature difference. - The relative contribution of the chiller and the storage varies during the discharge cycle. - Since both of them have the same entering water temperature, the control is easily achieved by setting a fixed leaving temperature. - A disadvantage is that there are many more interconnections between the chiller and the CTES to accommodate the changes in flow paths between charging and discharging cycles HCB 3- Chap 18B: TES

Modeling TES Using Effectiveness-NTU Method During initial stages of TES discharge, the outlet temperature is low while the cooling rate is high Gradually as the TES gets depleted, the temperature increases while the cooling rate decreases FIGURE 18.34 Outlet temperature and heat flow from an ice storage tank HCB 3- Chap 18B: TES

Effectiveness is close to unity during the initial stages of the discharge cycle. Gradually, the effectiveness decreases and reaches zero when the storage is fully discharged. Further, effectiveness varies with circulating fluid flow rate, and so a series of curves are necessary to model the effect of different fluid flow rates. These curves have to be generated for the specific design of the ice storage system being designed and can be generated either from manufacturer data, or from field tests, or from detailed modeling. FIGURE 18.35 Characteristic curves of an ice storage tank: effectiveness versus fraction of storage capacity discharged with high and low fluid flow rates HCB 3- Chap 18B: TES

Outcomes Knowledge of the diurnal and seasonal variability of building loads Understanding of the various cost components of the electric utility rate structure and the reasons for them Knowledge of how to calculate utility costs which include energy and demand components Understanding of the benefits offered by cool thermal energy storage (CTES) to both customers and utility Knowledge of the different ways of sizing chillers and CTES and be able to analyze simple idealized situations Familiarity of the operational principle and the thermal network analogues of the ice-on-coil storage element Understanding the notions of chiller-priority and storage-priority and the advantages/disadvantages of both types of series configurations Understand the parallel chiller-CTES system configuration Knowledge of how the effectiveness-NTU modeling approach can be used to model actual CTES systems HCB 3- Chap 18B: TES