MEBS 6008 Thermal Storage - I

1. MEBS 6008 Thermal Storage - I

2. Definition of Thermal Storage Thermal storage for HVAC applications can involve storage at various temperatures associated with heating or cooling. Energy may be charged, stored, and discharged daily, weekly, annually, or in seasonal or rapid batch process cycles.

3. Cool storage is associated storage receiving and accumulating cooling capacity output from the refrigeration plant, and the release cooling capacity to the load at some different time and rate. High temperature storage is associated with solar energy or high-temperature heating

4. Thermal storage may be an economically attractive approach to meeting heating or cooling loads if one or more of the following conditions apply: • Loads are of short duration • Loads occur infrequently • Loads are cyclical in nature • Loads are not well matched to the availability of the energy source • Energy costs are time-dependent (e.g., time-of-use energy rates or demand charges for peak energy consumption) • Utility rebates, tax credits, or other economic incentives are provided for the use of load-shifting equipment • Energy supply from the utility is limited, thus preventing the use of full-size nonstorage systems

5. Types of Some Thermal Storage Systems • Ice storage • Warm/cool water store • Fabric energy storage • Embedded pipework slab heating and cooling • Solar storage • ground source.

6. Fabric energy storage Fabric storage technologies, such as slab cooling passing air over the slab surface and via hollow slabs for limiting internal temperatures. Peak temperatures can be reduced and, in combination with other passive measures such as solar shading to provide comfortable conditions. Slab heating and cooling using embedded pipework Under floor heating and cooling by embedded pipework system can provide comfortable conditions in both heating and cooling modes.

7. Active Solar Storage Solar collector along with its associated pump to convert solar radiation into heat. The store which receives the heated water from the collector delivers heated water to the space heating heat exchanger. May contribute to the building's hot water requirements of between 6% and 12%.   Ground source Systems may be closed loop or open loop, and both types typically take water from a borehole, river or well. It is required to assess the characteristics of ground sources as this can vary widely. Heat pump selection needs to match these characteristics as well the energy requirements of the building.

8. Situations favorable to the adoption of cold thermal storage system Large daily temperature swing This is difference between daily max. DB temp. to that of daily min. (Compare Hong Kong and USA ) Large difference between off-peak energy charge and on-peak energy charge Compare HK - CLP & HKE and USA Large amount in demand charge What is demand charge? Small in ratio of Day-time cooling-demanding hours against night-time ice-making hours That is, ice is made during night at low energy charge

9. Typical Applications of Thermal Storage Churches, SportsFacilities, Horse racing, Coliseum • The load is short in duration and there is a long time between load occurrences, • They have a relatively large space-conditioning load for fewer than 6 hour per day and only a few days per week. • The relatively small refrigeration plant for these applications would operate continuously for up to 100 h or more to recharge the thermal storage.

10. Typical Applications of Thermal Storage Industrial Process Bakeries, 10 to 15 minutes of cooling every 2.5 hours to stop yeast fermentation Tire manufacture 2 minutes of cooling every 15 minutes to stop a vulcanizing process Dairies, 6 hours of cooling every 24 hours to cool milk after pasteurization.

11. Benefits of Thermal Storage Reduced Equipment Size If thermal storage is used to meet all or a portion of peak heating or cooling loads, equipment can be downsized to meet an average load rather than the peak load. Capital Cost Savings Capital savings can result both from equipment downsizing and from certain utility cash incentive programs. Even in the absence of utility cash incentives, the savings from downsizing cooling equipment can offset the cost of the storage. Cool storage integrated with low-temperature air and water distribution systems can also provide an initial cost savings due to the use of smaller chillers, pumps, piping, ducts, and fans. Storage has the potential to provide capital savings for systems having heating or cooling peak loads of extremely short duration.

12. Benefits of Thermal Storage (Cont’d) Energy Cost Savings The significant reduction of time-dependent energy costs such as electric demand charges and on-peak time-of-use energy charges is a major economic incentive for the use of thermal storage. Energy Savings Cool storage systems permit chillers to operate more at night when lower condensing temperatures improve equipment efficiency Storage permits the operation of equipment at full-load, avoiding inefficient part-load performance. Chilled water storage installations may reduce annual energy consumption for air conditioning by up to 12% Improved HVAC Operation Storage adds an element of thermal capacitance to a heating or cooling system, allowing the decoupling of the thermal load profile from the operation of the equipment. This decoupling can be used to provide increased flexibility, reliability, or backup capacity for the control and operation of the system.

13. Disadvantage of Cold Thermal Storage System • There will be distribution and storage vessel thermal losses that would not occur with a conventional system - pumping to both charge and discharge the store. •  Operation of chiller plant to produce ice requires a chiller capable of depressing its evaporating temperature to say, -6oC as opposed to the +6oC with conventional chiller plant. This reduces the chiller coefficient of performance (COP). • Ice storage systems use 15% more energy than conventional plant due to the lower operating COP and additional pumping energy requirements. • CIBSE Technical Memorandum states that the efficiency of ice storage relative to producing chilled water at 5oC is around 85% to 90%. • There would be inevitable heat loss in pipework and storage tank.

16. Unit of Cold Thermal Storage The ton-hour, or ton-h (kWh), is the unit of stored refrigeration. One ton-hour is the refrigeration or heat absorption of 12,000 Btu (3.516 kWh) performed by a refrigeration system during a 1-h period.

17. Church Example Load Profile The church operates for 3 hours on Sunday morning with a calculated instantaneous peak hour load of 40 ton. The load is steady for each of the 3 hours. The integrated cycle of cooling load is 120 ton-hours. In a conventional non-storage system, the plant would need to have a capacity to meet the instantaneous design load of 40 ton.

18. Church Example – A summary of plant capacity

19. Church Example -Day Cycle Using a day or 24 hour cycle and a partial storage mode The plant operates for the full cycle time of 24 hours, and the plant capacity reduces from the instantaneous load of 40 ton to 5 ton. The storage capacity in the partial storage mode is the integrated cycle capacity of 120 ton hours less the plant contribution of 15 ton-hours during the load period, or a net of 105 ton-hours. Assuming that the 3 hour load period was the on-peak period, shifting to a full storage mode reduces the plant output time by three hours. In full storage the plant operates for 21 hours, requiring a capacity of 5.71 ton to meet the integrated cycle load of 120 ton hours. The storage increases to the full capacity of 120 ton-hours since the plant would no longer contribute during the on-peak period.

20. Church Example -Day Cycle As a first alternative, compare the day-cycle partial storage to the non-storage. Day cycle partial storage affords a plant reduction of 35 ton or 87.5%. If plant cost is US$600 per ton, the saving is US$21,000. This saving is partially offset by the cost of providing the storage. If the cost of the storage is US$70 per ton hour, the storage cost is US$8400 yielding a net first cost saving of US$12,600

21. Church Example -Day Cycle For the second alternative, consider changing from day-cycle partial storage to day cycle full storage. In full storage, the load is not shared, storage requirement increases by 15 tons and the plant increases by 0.71 ton. Using the same per-unit costs as the first alternate, there is a first cost increase of US$1476. This additional capital expense would have to be supported entirely by the demand cost savings that could be realized by avoiding the electrical demand equivalent to the operation of the 5 ton plant in the "on-peak" period.

22. Church Example - Weekly Cycle Extending the church example to weekly cycle, the partial storage plant then operates for 168 hours at 0.71 ton to produce the integrated cycle load of 120 ton-hours. The smaller plant contributes less during the load period. Weekly cycle, therefore, increases the partial storage capacity to 117.8 ton-hours. For weekly cycle full storage, there is little change in the plant and the storage, with the plant increasing to 0.73 ton and the storage going back to the full 120 ton-hours. The weekly cycle plant, with a capacity of 0.71 ton capacity is only capable of producing the integrated cycle load, or weekly load of 120 ton-hours.

23. Church Example - Conclusion This plant has no reserve to meet any expansion of the load, or as is often the case, no reserve to handle an error in the original load calculation. Owners and operators are accustomed to meeting longer hours of operation simply by operating cooling equipment for a longer time. In the church example, using the minimum chiller on weekly cycle completely eliminates this reserve. The day cycle alternative, on the other hand, can at least meet a load of 120 ton-hours in each day.

24. Ice Storage and Chilled Water Storage Two thermal storage media are widely used for air conditioning systems: ice storage and chilled water storage. At a temperature difference of 10°C, 2.2 kg of chilled water can store 19 kJ of thermal energy, whereas 2.2 kg of ice can store 178 kJ. For the same stored cooling capacity, the storage volume for ice is only about 0.12 that of the chilled water. Note the density of water is 997 kg/m3 and the density of ice is 920 kg/m3. In addition, ice storage systems generally provide chilled water at a temperature of 1.1 to 1.7°C to produce cold supply air between 5.6 and 9.4°C. For chilled water storage system, the chilled water is typically supplied at discharge temperature between 4 to 7°C

25. Electricity Tariff Some electric utilities of foreign countries charge less during the night or weekend off-peak hours than during the time of highest electrical demand, which often occur on hot summer afternoons due to air-conditioning use. Electric rates are normally divided into a demand charge and a consumption charge. The monthly demand charge is based on the building’s highest recorded demand for electricity during the month. The consumption charge is based on the total measured use of electricity in kilowatt-hours (kWh) over a longer period and are generally representative of the utility’s cost of fuel to operate its generation facilities. In some cases, the consumption charge is lower during off-peak hours because a higher proportion of the electricity is generated by baseload plants that are less expensive to operate. Rates that reflect this difference are known as time-of-use billing structures.

26. Electricity Tariff The annual operating cost of each system being considered must be estimated, including both electrical demand and consumption costs to compare the costs of different systems,. To determine demand cost, the monthly peak demand for each system is multiplied by the demand charge and totaled for the year. The electrical consumption cost is determined by totaling the annual energy use for each system in kilowatt-hours and multiplying it by the cost per kilowatt-hour. For time-of-use billing, energy use must be classified by time-of-use period and multiplied by the corresponding rate.

27. Electricity Tariff In foreign countries (e.g. USA), there are certainly incentive in respect of different time charges that makes cold thermal storage favorable for consideration. Is there any special Rate offered by Power Companies on using ice-storage system in their premises ? The answer is currently yes for the Hongkong Electric but not for China Light and Power.

28. Hongkong Electric Co. – Maximum Demand Tariff Monthly Tariff Charges for the Supply Demand Charge (a minimum of 100 kVA ) For each of the first 400 kVA of $42.1/kVA in the month maximum demand in the month For each of the next additional kVA$41.1/kVA in the month of maximum demand in the month Energy Charge (Monthly consumption) For each of the first 200 kWh supplied per month per kVA of maximum demand (subject to a minimum of 100 kVA) in the month is charged at $1.023/kWh For each additional kWh supplied in the month is charged at $0.962/kWh

29. Hongkong Electric Co. – Commercial, Industrial and Miscell.Tariff Block Rate Tariff For each of the first 1500kWh supplied per month, the Net Rate is $1.093/ kWh From 1,501kWh and above, the Net Rate is $1.179/kWh

30. China Light and Power Tariff for Customer who have installed ice-storage air conditioning systems in their premises Monthly max. demand in kVA On-peak first 650kVA at $66.5 per kVA On-peak kVA over 650 at $63.5 per kVA Off-peak kVA up to the on-peak demand is $0.0 per kVA Off-peak kVA in excess of the on-peak demand is $26.0 per kVA Off-Peak period (9:00pm to 9:00am) Energy charge On peak - $0.689 per kWh for the first 200,000 kWh and $0.674 afterwards Off peak - $0.614 per kWh

31. Full Storage System All refrigeration compressors cease to operate during on-peak hours, and the building refrigeration load during that period is entirely offset by the chilled water supplied from the thermal storage tank

32. Partial Storage System – 100% Chillers in Operation Partial storage, or load leveling (in load-leveling mode) in which refrigeration compressors are operated at full capacity during on-peak hours

33. Partial Storage System – 50% Chillers in Operation Partial storage, or load leveling (in demand-limited mode) in which building electric demand limits only part of the refrigeration compressors operated

34. Low temperature air distribution and ice storage - part 1 Low temperature distribution systems supply air to the occupied zone at between 4oC and 10oC, in contrast to most conventional air distribution systems, which supply at between approximately 14oC and 18oC. Low temperature supply air systems are often used with an ice storage system to take advantage of the low chilled water temperature. The supply air temperature achieved depends on the chilled water temperature and the characteristics of the cooling coil, the supply fan heat gain, air leakage paths, insulation condition, ductwork length, etc.

35. Low temperature air distribution and ice storage - part 2 ASHRAE suggests a differential of 3 to 6K between chilled water supply temperature to the coil and air temperature leaving the coil. Leakage from cold air ducts must be considered as this can cause condensation problems. Air handling units must be insulated from the mixed air section to the supply air outlet. As the temperatures involved are lower than in conventional applications, the performance of the diffusers must be assessed accordingly to prevent cold air dumping.

36. Low temperature air distribution and ice storage- part 3 The use of cold air distribution technology has a number of benefits including: Reduced mechanical system costs - smaller air handling units, ducts, pumps, and coils can be used to achieve the same cooling to the space. Air and water distribution costs can be reduced by 14-19% when the supply air temperature is reduced from 13oC to 7oC Decreased floor to floor height requirements due to smaller ducts improved comfort due to lower relative humidity in the occupied zone. The lower supply air temperature reduces supply air moisture content which reduces relative humidity in the occupied area Reduced fan energy consumption - reduced air flow rate requires smaller fans. AHU energy consumption can be reduced by 20-30%. Increased cooling capacity for existing distribution systems - this is an ideal solution where internal heat gains have increased.

37. Example of reduction in equipment size– Non-storage System If the 6120 kWh load is met by a non-storage air-conditioning system, a 660 kW chiller is required to meet the peak cooling demand.

38. Example of reduction in equipment size– load-leveling partial storage system An 255 kW chiller meets the demand. The design-day cooling load in excess of the chiller output (3060 kWh) is supplied by the storage. The cost of storage approximates the amount saved by downsizing the chiller, cooling tower, electrical service, etc. Load-leveling partial storage is often competitive with non-storage systems on an initial cost basis.

39. Example of reduction in equipment size – full storage system The entire peak load is shifted to the storage. A 360 kW chiller is required. The size of the chiller equipment may be reduced, but the total equipment cost including the storage is usually higher for the full storage system than for non-storage systems. Although the initial cost is higher than for the load-leveling system, full storage offers large reduction on operating costs because the entire chiller demand is shifted to the off-peak period.

40. Control Strategies A thermal storage control strategy defines how the system is controlled in a specific operating mode. The control strategy defines what equipment is running and the actions of individual control loops, including the values of their setpoints, in response to changes in load or other variables.

41. Control Strategies One Charging the Storage Control strategies for charging are generally easily defined. Typically the generation equipment operates at full capacity with a constant supply temperature setpoint and a constant flow through the storage. This operation continues until the storage is fully charged or the period available for charging has ended. Under this basic charging control strategy, the entire capacity of the equipment is applied to charging storage.

42. Control Strategies Two Charging Storage while Meeting Load A control strategy for charging storage while meeting load may also operate the generation equipment at its maximum capacity. The capacity that is not needed to meet the load is applied to charging storage. Depending on the system design, the load may be piped either in series or parallel with storage under this operating mode. Some systems may have specific requirements for the operating strategy in this mode. For example, in an ice storage system with a heat exchanger between glycol and water loops, the control strategy may have to address freeze protection for the heat exchanger.

43. Control Strategies Three Meeting Load from Discharging Only. A control strategy by discharging only (full storage or load shifting operation) is also straightforward. The generating equipment does not operate and the entire load is met from storage. Control Strategies Four Meeting Load from Discharging & Direct Equipment Operation These strategies must regulate what portion of the load at any time will be met from storage and what proportion will be met from direct generation. These partial storage strategies have been mostly developed for and applied to cool storage. Three common control strategies are chiller priority, storage priority, and constant proportion or proportional.

44. A chiller priority control strategy operates the chiller, up to its available capacity, to meet loads. Cooling loads in excess of the chiller capacity are met from storage. If a chiller demand limit is in place, the available capacity of the chiller is less than the maximum capacity. Chiller priority control can be implemented with any storage configuration. However, it is most commonly applied with the chiller in series upstream of storage. A simple method of implementing chiller priority control is to set the chiller setpoint and the temperature downstream of storage to the desired chilled water supply temperature. When the load exceeds the chiller capacity, the supply temperature exceeds the setpoint, and some flow is diverted through storage to provide the required additional cooling.

45. A storage priority control strategy meets the load from storage up to its available discharge rate. If the load exceeds this discharge rate, the chiller operates to meet the remaining load. If a storage discharge rate limit is in place, the available discharge rate is less than the maximum discharge rate. A storage priority strategy must ensure that storage is not depleted too early in the discharge cycle. Failure to properly limit the discharge rate could cause loss of control of the building or excessive demand charges or both. Load forecasting is required to maximize the benefits of storage-priority control. Simpler storage-priority strategies using constant discharge rates, or predetermined discharge rate schedules have been used.

46. A constant proportion or proportional control strategy divides the load between chiller and storage. The load may be divided equally or in some other proportion. The proportion may change with time in response to changing conditions. A limit on chiller demand or storage discharge may be applied.

47. Demand-limiting control It may be applied to any of the above control strategies (chiller priority, storage priority, and constant proportion or proportional). This type of control attempts to limit the facility demand either by setting a maximum capacity above which the chiller is not allowed to operate or by modulating the chiller set-point. The demand limit may be a constant value or it may change with time in response to changing conditions. Demand limiting is most effective when the chiller capacity is controlled in response to the facility demand at the billing meter. In such cases, the chiller capacity is controlled to keep the total demand from exceeding a predetermined facility demand limit. A simpler approach, which generally achieves lower demand savings, is simply to limit chiller capacity or the chiller’s electric demand without considering the total facility demand. Operating strategies that seek to optimize system operation often recalculate the demand limit and discharge limit on a regular basis during the discharge period.

48. Electricity Tariff Applicable utility rates and system efficiency in various operating modes determine the selection of a control strategy. If on-peak energy cost is significantly higher than off-peak energy cost, the use of stored energy should be maximized and a storage priority strategy is appropriate. If on-peak energy is not significantly more expensive than off-peak energy, a chiller priority strategy is more appropriate. If demand charges are high, some type of demand-limiting control should be implemented.

49. Refrigeration Design For the most part, chillers in chilled water storage systems operate at conditions similar to those for non-storage applications. However, a greater percentage of the operating hours occur at lower ambient temperatures; special consideration should thus be given to providing a condensing temperature that maintains compressor differential. The lower suction temperature necessary for making ice imposes a higher compression ratio on the refrigeration equipment. Positive displacement compressors (e.g., reciprocating, screw, and scroll compressors) are usually better suited to these higher compression ratios than centrifugal compressors.

50. Cooling Load and Cold Storage - 1 Cool storage systems generally have less capacity to recover than non-storage systems if design loads are exceeded. For example, in an application for which the 2.5% design temperatures would be used for a non-storage design, the 1% values are recommended for a cool storage design. Designers may elect to use less extreme design weather conditions for full storage systems, since a full storage system can fall back to partial chiller operation if design loads are exceeded. Load profiles must be calculated for the entire design charge-discharge cycle of the cool storage system. The most common cycle is 24 h long, but weekly cycles are also applied when appropriate. Longer or shorter cycles are also possible for certain applications.