1. 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

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

3. 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

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

5. 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

6. 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

7. 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

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

9. 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

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

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

12. APS Electric Utility
HCB 3- Chap 18B: TES

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

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

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

21. FIGURE 18.33 Different stages of charging an ice on coil system
Somewhat complex
to model due to
constrained growth
FIGURE 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

22. Outdoor Application
HCB 3- Chap 18B: TES

23. 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

24. Example : Idealized comparison of different CTES sizing options
Building whose cooling load in tons is shown in 2nd column of Figure 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

25. Figure 18.30
HCB 3- Chap 18B: TES

26. HCB 3- Chap 18B: TES

27. 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

28. 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

29. 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

30. 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 Outlet temperature and heat flow from an ice storage tank
HCB 3- Chap 18B: TES

31. 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 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

32. 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