Thermal Storage Systems - One

1. MEBS6008 Environmental Services II http://www.hku.hk/mech/msc-courses/MEBS6008/index.html Thermal Storage Systems - One Ir. Kelvin Tam Department of Mechanical Engineering The University of Hong Kong

2. The Course • MEBS6008 Environmental services II • Thermal Storage Systems - One • Thermal Storage Systems - Two • Sea Water Heat Rejection System & Heat Recovery Systems • Heat Pump Systems • Acoustic Treatment and Vibration Control - One • Acoustic Treatment and Vibration Control - Two • Assessment: 100% by examination

3. Review to Environmental Services I • Your Constructive Feedback (after lecture) is preferred to any comments made an the evaluation form. • Examination of Environmental Services I (your feedback) 3

4. Content • Why Bother Thermal Storage • Definition of Thermal Storage • Types of Thermal Storage Systems • Situations favour the use of Thermal Storage Systems • Unit of Cold Thermal Storage • Cold Storage Media • Typical Applications of Thermal Storage • Benefits of Cold Thermal Storage • Disadvantages of Cold Thermal Storage System • Operating Strategies • Examples of reduction in equipment size by storage • Chiller priority, storage priority & constant proportion • Systems Schematics • Church Example

5. Why Bother Thermal Storage ? • Primary energy source -Hydro, Gas, Coal and Nuclear fuels transformed directly into Electricity as a power source for industrial and household appliances. • In principle, electricity generation has to be balanced with the exact time of the consumption to satisfy the fluctuating demand at the lowest possible cost. Thermal Storage - 1

6. Why Bother Thermal Storage ? • Fluctuating seasonal and specific time demands outside their control and • The essential specific running time requirement of electricity generation plants which do not necessarily match the demand. • Utility companies generate electricity using different types of primary energy sources to offset peak demands and a typical UK electricity generation pattern. Thermal Storage - 1

7. Why Bother Thermal Storage ? • Almost every modern society has a mid-day or late evening peak electricity demand. • This essential demand force utility companies to build new additional peak demand power stations -> considerable investment that operate only during peak demand periods and shut down the rest of the time. • They use expensive primary energy sources and are subject to the standard cost of maintenance, consequently production cost per kWh is 3-4 times higher than the standard base load electricity production cost. Thermal Storage - 1

8. Definition of Thermal Storage Thermal storage for HVAC applications 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. Thermal Storage - 1

9. Types of Thermal Storage Systems • Cold storage • Fabric and slab energy storage • Solar storage • ground source • Packed Rock Beds • Low Temperature CO2 Storage System • Thermochemical Energy Storage Thermal Storage - 1 9

10. Types of Thermal Storage Systems Cool storage Storage receiving and accumulating cooling capacity output from the refrigeration plant. Release cooling capacity to the load at some different time and rate. Fabric & Slab energy storage Building materials absorbed heat/ cooling during a particular period and release it at another period. Thermal Storage - 1

11. Types of Thermal Storage Systems 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%.   Thermal Storage - 1

12. Types of Thermal Storage Systems 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. Thermal Storage - 1 12

13. Types of Thermal Storage Systems Packed Rock Beds Apacked rock bed utilises the available thermal energy by means of circulating through a packed rock bed to add heat to or remove heat from the system for charging and discharging respectively. The energy can be transferred from a fluid but the most common systems utilise air due to the high heat transfer coefficient between air and rock. Thermal Storage - 1

14. Types of Thermal Storage Systems Low Temperature CO2 Storage System Carbon Dioxide offers the most compact latent heat storage system due to the commercially obtainable triple point which allows the utilisation of a single substance as static latent heat of fusion storage. Carbon Dioxide can be stored at it‘s triple point of -57 Deg C and 518 kPa with solid fraction of 70-80 % by mass and the system can provide 140 kJ/kg thermal storage capacity within the required volume of 166.6 MJ/m3. Thermal Storage - 1

15. Types of Thermal Storage Systems Thermochemical Energy Storage Recent research shows that various alcohols and ketones are potentialthermochemical storage media but due to the relative cost and complexity, no commercially viable systems have yet emerged. Typical examples are the mixture of Sulphuric Acid and water, and alternatively Sodium Hydroxide and water. Systems in which the water is separated by the heat input to the mixture and as soon as the two substance are mixed, the chemical reaction of the substances liberates heat. Thermal Storage - 1

16. Situations favour the use of Thermal Storage Systems The storage systems are most likely to be cost-effective in situations :- • A facility's maximum cooling load is much greater than the average load; An existing tank is available; • Limited electric power is available at the site; • Backup cooling capacity is desirable; • Loads are of short duration, infrequently, 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) Thermal Storage - 1

17. Situations favour the use of Thermal Storage Systems • Large daily temperature swing Variations between day and night time ambient temperatures reaches 10-15oC Heat rejection equipment i.e. Air Cooled Chiller and Air Cooled Condenser operate more efficiently. • The head pressure (Condensing Pressure) changes proportionally with the ambient temperature. • The lower the ambient temperature, the lower the condensing pressure A typical example of a cooling system operating data Vs condensing temperature Thermal Storage - 1

18. Situations favour the use of Thermal Storage Systems • Cold air distribution would be advantageous It may be adopted in case of ice storage system. Low temperature distribution systems supply air to the occupied zone at 4oC -10oC. • 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. Thermal Storage - 1

19. Situations favour the use of Thermal Storage Systems • Cold air distribution would be advantageous (Cont’d) • 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 the conventional applications, the performance of the diffusers prevent cold air dumping. Thermal Storage - 1

20. Situations favour the use of Thermal Storage Systems • Cold air distribution would be advantageous (Cont’d) 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 RH in the occupied zone. Reduced fan energy consumption - reduced air flow rate requires smaller fans. AHU energy consumption reduced by 20-30%. Increased cooling capacity for existing distribution systems - an ideal solution where internal heat gains have increased. Thermal Storage - 1

21. Situations favour the use of Thermal Storage Systems • An existing cooling system is being expanded; Any future or additional cooling/heating demand can be easily satisfied by means of changing the thermal storage strategy for the system. • The additional capacity can be provided by shifting from a full storage to a partial storage or even weekly storage system depending on the required additional capacity over the existing capacity limits. • Utility rebates, tax credits, or other economic incentives are provided for the use of load-shifting equipment Thermal Storage - 1

22. Situations favour the use of Thermal Storage Systems • The utility rate structure has high demand charges or a high differential between on-and off-peak energy rates; Some electric utilities of foreign countries charge less during the night or weekend off-peak hours than during the time of highest electrical demand 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. Thermal Storage - 1 22

23. Situations favour the use of Thermal Storage Systems In England, the off-peak period is between 12.00 pm and 7.00 am at an average rate of average 2.68 p/kWh against the standard charge of 7.35 p/kWh. In the USA, due to the large air conditioning load this structure has been generally divided into Winter and Summer charges but still offers similar incentives : - Off-peak cooling running costs are almost half of those of a conventional system Thermal Storage - 1 23

24. Situations favour the use of Thermal Storage Systems Is there any special Rate offered by Power Companies on using ice-storage system in their premises in HONG KONG ? Thermal Storage - 1

25. Situations favour the use of Thermal Storage Systems Hong Kong Electric – Maximum Demand Tariff – Demand Charge Thermal Storage - 1 25

26. Situations favour the use of Thermal Storage Systems Hong Kong Electric – Maximum Demand Tariff - Energy Charge "FCA" means Fuel Clause Adjustment. Thermal Storage - 1 26

27. Situations favour the use of Thermal Storage Systems China Light & Power – Bulk Tariff – Demand Charge China Light & Power – Ice Storage Tariff – Demand Charge Thermal Storage - 1 27

28. Situations favour the use of Thermal Storage Systems China Light & Power – Ice Storage Tariff – Energy Charge China Light & Power – Bulk Tariff – Energy Charge Thermal Storage - 1

29. 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. Thermal Storage - 1

30. Cold Storage Medium - Chilled water Chilled-water storage systems They use the sensible heat capacity of waterto store cooling capacity. They operate at temperature ranges compatible with standard chiller systems and are most economical for systems greater than 2,000 ton-hours in capacity. The capacity of a chilled-water thermal energy storage system is increased by storing the coldest water possible and by extracting as much heat from the chilled water as practical (thus raising the temperature of the return water). Thermal Storage - 1

31. Cold Storage Medium - Ice Ice thermal storage systems They use the latent heat of fusion of water to store cooling capacity. Storing energy at the temperature of ice requires refrigeration equipment that can cool the charging fluid (typically, a water/glycol mixture) to temperatures below the normal operating range of conventional air-conditioning equipment. Special ice-making equipment or standard chillers modified for low temperature service are used. The low temperatures of the chilled-water supply allow the use of low-temperature air distribution, meaning smaller fans and ducts are needed. Thermal Storage - 1

32. Cold Storage Medium-Eutectic salts. Eutectic salts They are also known as phase-change materials. They use a combination of inorganic salts, water, and other elements to create a mixture that freezes at a desired temperature. The material is encapsulated in plastic containers that are stacked in a storage tank through which water is circulated. The most commonly used mixture for thermal storage freezes at 8.3°C, which allows the use of standard chilling equipment to charge storage. Thermal Storage - 1

33. Cold Storage Medium – a Comparison Chilled water systems They require the largest tanks, but they can easily interface with existing chiller systems. Ice systems They use smaller tanks and offer the potential for the use of low- temperature air systems, but they require more complex chiller systems. Eutectic salts They can use existing chillers but usually operate at the warmest temperatures. Thermal Storage - 1

34. Cold Storage Medium – a Comparison Thermal Storage - 1

35. Cold Storage Medium – a Comparison

36. Typical Applications of Thermal Storage Churches, SportsFacilities, Horse racing, Coliseum, theatres • 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. Thermal Storage - 1

37. Typical Applications of Thermal Storage Industrial Process - food processing, dairy, brewery, processing and gas turbine air inlet gas cooling for industrial applications. 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. Thermal Storage - 1

38. Benefits of Cold Thermal Storage Reduced Equipment Size Equipment can be downsized to meet an average load rather than the peak load. Chillers for thermal storage applications are generally 30-60% smaller than the conventional system chillers (longer running periods and large latent heat storage capacity). Chiller(s) run most of their expected life span running at full load during the charging mode and supplement the operation for partial storage strategy. Thermal Storage - 1

39. Benefits of Cold Thermal Storage Capital Cost Savings Due to equipment downsizing and utility cash incentive programs. Downsizing cooling equipment offset the cost of the storage. Cool storage integrated with low-temperature air and water distribution systems provide an initial cost savings (smaller chillers, pumps, piping, ducts, and fans.) For systems having heating or cooling peak loads of extremely short duration. Thermal Storage - 1

40. Benefits of Cold Thermal Storage Energy Cost Savings The significant reduction of time-dependent energy costs such as electric demand charges and on-peak time-of-use energy charges Energy Savings Chillers operate more at night with lower condensing temperatures-> improve efficiency Operation of equipment at full-load, avoiding inefficient part-load performance (may reduce annual energy consumption by up to 12%) Improved HVAC Operation Decoupling of the thermal load profile from the operation of the equipment. =>increased flexibility, reliability, or backup capacity for the control and operation. Thermal Storage - 1

41. Benefits of Cold Thermal Storage Full Stand-by Capacity The stored thermal energy can provide reasonable safety periods for any regular and/or emergency repair works without disturbing the system. Full Stand-by capacity becomes quite essential for industrial and continuous space conditioning applications. Allow Free Cooling In a climate where the night ambient temperature drops below the thermal storage temperature, the storage system can be charged by means of free cooling. Thermal Storage - 1

42. Disadvantage of Cold Thermal Storage System 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%. Inevitable heat loss in pipework and storage tank. Thermal Storage - 1

43. Full-storage operating strategy A full-storage, or load-shifting, strategy shifts the entire on-peak cooling load to off-peak hours. This strategy is most attractive where on-peak demand charges are high or the on-peak period is short. Thermal Storage - 1 43

44. Partial storage load: Load-leveling operating strategy Chiller runs at its full capacity for 24 hours on the design day. Load < chiller output, surplus cooling is stored. Load > chiller output, additional requirement is discharged from storage. Most effective : Peak cooling load >> the average load. Thermal Storage - 1 44

45. Partial-storage load: demand-limitingoperatingstrategy The chiller runs at reduced capacity during on-peak hours Controlled to limit the facility's peak demand charge. Demandsavings and equipment costs : Demand-limiting system > load-leveling system Demand-limiting system < full-storage system. Thermal Storage - 1 45

46. Example of reduction in equipment size – Non-storage System kWH from 6am to 6pm = 6120 kWh Storage : Chiller Capacity: 660 kW (peak demand). Thermal Storage - 1

47. Example of reduction in equipment size– load-leveling partial storage system The design-day cooling load > 255kW (that is, 3060 kWh) from storage. Chiller :255 kW The cost of storage < saving by downsizing equipment (i.e. initial cost). Thermal Storage - 1

48. Example of reduction in equipment size – full storage system The entire peak load from storage. Chiller: 360 kW Initial cost of full storage> load-leveling system, full storage offers large reduction on operating costs (demand shifted to off-peak period). Thermal Storage - 1

49. Chiller priority, storage priority & constant proportion Chiller priority controlstrategy Cooling Load < Chiller capacity : operates the chiller, up to its available capacity. Cooling Load < Chiller capacity : Cooling capacity from storage. Demand limit : Available capacity of chiller < maximum capacity. Load > the chiller capacity => supply temperature > setpoint => some flow is diverted through storage to provide required additional cooling. Storage priority control strategy Load < discharge rate: from storage up to its available discharge rate. Load > discharge rate: Chiller operates to meet the remaining load. Storage discharge rate limit : available discharge rate < maximum discharge rate. Ensure storage not depleted too early in the discharge cycle. Else loss of control of the building or excessive demand charges or both. Need correct load forecasting. Thermal Storage - 1

50. Chiller priority, storage priority & constant proportion A constant proportion control strategy It divides the load between chiller and storage. Load : 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. Thermal Storage - 1