Monthly Engineering Horizons

THE BENEFITS OF ICE THERMAL STORAGE SYSTEMS PART-2

THE BENEFITS OF ICE THERMAL STORAGE SYSTEMS PART-2

April 02
01:30 2020

By: Muhammad Saghir Khan, Chief Executive, Energy Associates Building Services Consultants Inc. Ontario, Canada

  1. APPLICITY OF THERMAL STORAGE SYSTEMS

Thermal energy storage can be used in virtually any building; the merits are compelling in the right situations. The main issues are the type of storage system and the amount of cooling load to be shifted.  Before embarking on a cool storage project, however, one should consider the field observation guidelines below. In addition, alternative approaches should be considered for each project., A major and total energy savings impacts  due to  application of Thermal Storage Systems  on power plants which reduces the fuel or energy required at the source by changing the time at which KWhs  of electricity  are used , which is going to explained in foregoing discussions. (5)

5              TOTAL ENERGY SAVINGS IMPACTS OF ICE THERMAL STORAGE SYSTEMS.

The general belief is that TES reduces the fuel or energy required at the source by changing the time at which KWhs of electricity are used.

The amount of cooling desired at various hours of the day in a typical commercial office building in Pakistan is shown in Figure no.1-1In a conventional cooling system the electricity use follows the demand for cooling- since chiller must run to cool the building.

Air conditioning (and industrial process cooling) makes up almost a third of the aggregate electricity demand on Power Generation systems apart from Hydel power during peak summer period.

In order to keep over –all electricity costs down, the electric utilities run their most economic and typically most efficient “ base load” power plants as much as possible. Other power plants are somewhat less efficient. These “ Intermediate” power plants see limited use during the day. Finally, plants with highest operating costs and typically lowest efficiency are mainly used during the few “on-peak” hours.  Hence, they are called “peak” load power plants, as shown in figure 1-2

The cost to produce a kWh of electricity is highest during these on-peak hours, because the least efficient power plants run during the on-peak hours, the costs of generating the electrical “energy” are higher during those hours. This leads to a situation in which electricity users can reduce their electricity costs under Time-of use rates if they can reduce their peak electricity use. TES provides electricity users that opportunity, as summarizes bellow

  • An Electric utility‘s costs and prices are much higher during on-peak hours.
  • An Energy user who uses less on –peak electricity can save significant amount of money.
  • Thermal Energy storage reduces on-peak electricity use.

There are typically two basic strategies for using a TES to reduce on-peak electricity use as Figure 1.3 shows. The first strategy, “full storage” sizes the chiller and storage tank so that the chiller does not run at all during peak hours on the hottest days. In contrast, the “partial storage” strategy sizes the chiller and storage tank so that a smaller chiller runs continuously on hot days. The main advantage of the “Full Storage” system is that it minimizes electricity costs. The main advantage of the “partial storage” system is that a smaller chiller and smaller storage tank reduce the capital costs of the TES system. The pay back period for cool storage systems depends on the demand charges differential and cooling requirements. Typical installations show a 5  – 10 years payback {EPRI, 1994}

Figure 1.3   Electricity use for cooling with a TES system

This study tests that belief by quantifying the source energy impact of TES, as pointed out in EPRI

Study that there are two Methodologies for determining source energy savings are defined which are as follow

  • Incremental Energy Method
  • Marginal Plant Method

The Incremental energy method is more consistent with other planning and evaluation methods used by the California Public Utilities Commission.

For this study purpose we will calculate the source energy savings by “Incremental Energy Method.”

5.1          Source Energy Savings

The Incremental energy method has been defined to calculate the source energy savings estimates of TES programs. In particular, the “marginal cost of a kWh” is often called the “Marginal energy cost”. This cost (in $/kWh) for each five time periods equals the cost of fuels (in $/BTU  or usually $/million BTUs) multiplied by the average heat rate or more precisely Incremental Energy Rate (in BTU/kWh). Since the crude oil is almost always the fuel of the Marginal plant, in Pakistan imported Crude oil, dividing the marginal energy cost for each of the 5 time periods by the price of crude oil yields the average Incremental Energy Rate (in BTU/kWh ) for each  of the time periods. There are three components in determining the source energy use. The first component is the number of kWhs at the energy user’s site. The second component is the fuel used at the power plant to generate the KWhs for use at the energy user’s site. The third component is the energy used to get the electricity across the power lines from the power plants to the user. In particular, there is energy lost due to resistance in the power lines (Line losses) for example, to get 1.00 kWh of electricity delivered to the energy user’s site, 1.1 kWhs may need to be input into the power lines at the power plant. This amounts to 10% line losses.  Moreover, an important factor in this TES study is that these line losses vary across the five time periods. In particular, line losses are highest when the lines are more fully loaded and when ambient temperature is hotter. Both of these factors lead to line losses being higher during the summer, on –peak period. Therefore TES saves energy by shifting electricity use to times of lower line losses.

SOURCE (EPRI, 1994)

Final source energy saving formula (SOURCE): California Public Utilities Company standard practice manual.

Table 2-1

 
 

TES Source energy savings = å5 (kwh savings)1 x Incremental energy rate)1

x (line loss factor) i = 1

 

 

 

To clearly accommodate the line loss factors, the source energy savings formula has been established and which show that incremental energy rate is broken into two components, – incremental energy rate at the Power Plant Source and line loss factor to get the energy to the customer site.

In addition to this, one simplification is being made to make the source energy savings are normalized and are assumed to yield no net kWh savings at the site.

This is illustrated in table 2.2 for example, a kWh “Saved” in the summer on-peak period was assumed to be shifted to the mid-peak and off-peak period (where it shows up as increased kWh use).

The same approach was used for the winter in which all kWh savings during the mid-peak period were assumed to be shifted to off-peak.

However, the size of the number of kWh “saved” during the winter mid-peak was varied to reflect the fact that for different buildings in different locations the winter mid-peak kWhs.  For example, assume a building site without TES would normally use 30% of its annual cooling kWh during the summer on-peak period and 40% of its annual cooling kWh during the winter mid-peak period.  Also assume the site then installed a full storage TES system and shifted all summer on-peak kWhs and all winter mid-peak kWhs.

In this situation, the “X” in table 2-2 would become 1.33 (=40% / 30%).  If the building used a partial storage system, then may be only 2/3’s of the summer on-peak kWhs could be shifted.  Then the X in the table 2-2 would become 2(=40% / 30% * 2/3).  In this study later in this section, the value of X was varied to reflect a range of building types (e.g. large office, small office and hospital), TES storage systems (full vs. partial) and utility services.

 
 

Table 2-2

Typical TES kWh Shifting Across Time Periods

SOURCE: EPRI,  1994

 

Summer

On-peak                 -1.00kWh

Mid-peak               -25 kWh

Off-peak                -75 kWh

 

Winter

Mid-peak               -x kWh

Off-peak                x kWh

 

 

One of the advantages of making the assumption of no net kWh savings in table 2-2 is that it allowed this study to separate the source energy savings analysis from the site (or kWh) savings for the forthcoming section 2.3 will be discussing the site energy savings.

The second term of concern in the formula in Table 2-1 is the Incremental Energy Rates by time period. Table 2-3 shows SCE’s projected Incremental Energy Rates at the power plants for 1995. The summer on- peak and winter mid – peak Incremental Energy Rates include fuel use for “ unit commitment.”  Note that the Incremental Energy Rate for the summer mid-peak and off- peak are 38% and 46% less, respectively, than the Incremental Energy Rate of the winter off-peak period is 31 % less than the winter mid peak period. This number mean TES can save significant source energy by shifting kWhs in summer and in winter.

The third term of concern in the formula in Table 2-1 is the line loss factors.  Table 2-4 shows the relative loss factors used.  (PG&E 1995) It shows that the off – peak line losses at the secondary voltage average about 5% lower than the line losses during the summer on peak.

 
 

Table  2-3

Relative SCE “Incremental Energy Rates” by Time Period

 

                                                Summer                 Winter

                On-peak                 14251                    —

Mid-peak               8818                       10714

Off-Peak                7647                       7419

 

  % Difference by Time Period

                                                Summer                 Winter

On-peak                 —                             —

Mid-peak               38%                        —

Off-peak                46%                        31%

 

Source: SCE “Marginal Cost Exhibit documentation for CPUC in General Rate Case for test year 1995. Revised March 1995.

 

 

 

The effect of the differences in Incremental Energy Rate and line loss factors can be combined.  Table 2-5 shows that the combined effect yields a source energy savings of 49% for each summer on-peak kWh that is shifted to the off- peak.  (The % savings of table 2-3 and 2-4 are not additive because of the multiplicative relationship of energy used at the power plant and source energy).

Table 2-5

SCE Source Energy Use % Differences

 

                                                Summer                 Winter

 

                On-peak                 —                             —

                Mid-peak               40%                        —

                Off-peak                49%                        33%

SOURCE: CEC Thermal Energy Storage Systems Collaborative

 

Air conditioning engineers also will find it useful to characterize this information in an alternate way. The source energy savings can be characterized as a % of the source energy required to meet the total annual cooling load. This % can be commuted by multiplying, % source energy savings per kWh of annual cooling load = (% source energy savings per kWh shifted) x (% of annual kWh shifted by TES)

The first multiplicand – (% source energy savings per kWh shifted) – comes from figure 2-3.

Figure 2-3

SCE Source Energy Savings per kWh shifted

Using the Incremental Energy Method

Ratio of winter kWh shifted to summer kWhs shifted

Multiplying these ranges of %, s together yields the following range of % source energy savings per kWh of annual cooling:

  • 14%, typically for organizations with 24 hour a day cooling and partial storage.
  • 28%, typically for small office buildings with packaged air conditioners replaced with full storage.

In summary, the incremental Energy method for Edison reveals significant sources energy savings from shifting kWhs of electricity with TES. The savings calculations from the Incremental Energy method are the recommended savings estimates, which were 36-43 % per kWh shifted for normal buildings.

5.3          Site Energy Savings

The Site Energy savings, running the main equipment in full load operation can make that certain energy savings. TES systems can provide enhanced efficiency is by having the chillers (and their supporting pumps and fans) run fully loaded most of the time at their peak efficiency.

As noted in the previous section, the chillers and support equipment of conventional cooling systems must run whenever the building occupants want cooling. The chiller system capacity is sized for the peak (or design) cooling day. However, most of the year the chiller system does not operate near peak cooling conditions in Pakistan, as Figure 2-4 shows. In fact, about half of the year a typical chiller system operates at less than 30% of capacity.  At such low capacity loading, the energy efficiency of a conventional chiller system decrease – or its energy intensity (kWh/ton – hour of cooling) substantially increases, as Figure 2-5 shows. Thus, much of the year a conventional chiller system can operate an energy intensity that is 2-4 times higher than its design intensity. 

In well-designed TES systems the chiller system almost always operates fully loaded. By having a set of chiller (primary) pumps that operate separately from the distribution system (secondary) pumps, the chiller and its pumps can run at efficient fully loaded levels. Thus, the more frequently that the cooling load is less than design capacity, then the better that TES looks compared to conventional cooling systems.

There are many successful TES systems operating today demonstrate that the technology can provide significant benefits. However, many cool storage systems have failed to perform as pre- diced because they did not meet the criteria for applicability cited below.

  • The maximum cooling load of the facility is significantly higher than the average The electric utility rate structure includes high demand charges, ratchet charges, or a high differential between on- and off-peak energy rates. The economics are particularly attractive where the cost of on-peak demand and energy is high.
  • Electric power available at the site is limited. Where expensive trans- formers or switchgear would otherwise have to be added, the reduction in electric demand through the use of cool storage can mean significant
  • Cold air distribution can be used, is necessary, or would be Cool storage technologies using ice permit economical use of lower- temperature supply water and air. Engineers can downsize pumps, piping, air handlers, and ductwork, and realize substantial eductions in first cost.
  • Backup or redundant cooling capacity is desirable. Cool storage can provide short term backup or reserve cooling capacity for computer rooms and other critical applications
  • An existing cooling system is being expanded. The cost of adding cool storage capacity can be much less than the cost of adding new

5.4          Country wise Potential of Source & Site Energy Savings

( i )          Source Energy Savings

To provide some perspective on this value of such savings within country consider the following.

Today the electricity use for air conditioning in Pakistan is about 3500 GWh.,By 2020 it will be close4500 GWH,. Which equals the electricity use today for all Air Conditioning customers served by local power companies.

If TES achieved an 10 -15 % market penetration by 2020, then about450 – 675 GWH equivalents of source energy could be saved. Based on average forecasts, this is enough source energy savings to supply about a fifth of all new air conditioning growth in the next decade- even if TES saves no kWhs of electricity.

In summary, TES can provide significant source energy savings to kingdom in the next decade if aggressively promoted.

(ii )          Site Energy Savings

The aggregate potential site efficiency savings can be estimated from this information. In particular assumed that there was about 3500 GWhs of air conditioning load in 2020. Suppose the 15 % potential site efficiency could be achieved at 25 % of the installations. Then 15% of the electricity required to supply new air conditioning load in the next decade could come from these site efficiency improvements. If site efficiency and source energy savings are combined, then 20% penetration of TES can supply over a third of the energy needs of new air conditioning in the next decade.

In summary, the TES community is evolving. There are an increasing number of instances where site efficiency improvements have been achieved along with load shifting.

5.5          Country wide Economic Development / Competitive Impacts

TES provides several economic benefits to electricity. The first major benefit is lower operating or production costs. As pointed out in Incremental Energy method used in the Source Energy calculations is also used to calculate the marginal energy costs. Thus, the utility is not only reducing its source energy requirements by 20-43% per kWh shifted with TES, it is also reducing its marginal energy costs of producing a kWh by 20-43%. These saving are quite significant, as pointed out in previous section.

A second major benefit is improving the capital asset utilization of electric suppliers. The electricity supply industry is one of the most capital-intensive industries in the Country.

TES provides the capability to improve the load factor of many commercial facilities by 30-50%. That means an electricity supplier can reduce its capital intensity (expenditures) in serving such customers by 30 – 50 %.  Those are huge capital savings.

5.6          Pakistan wise, Potential of Economic Savings

The potential aggregate peak demand savings of TES is significant. The potential new growth in air conditioning load in the next decade is about 500 MW. Air conditioning is currently about 3900 MW or about a 1/5 Th. of the total peak demand in Pakistan. TES in 10-15 % of buildings by 2018 could reduce air conditioning load by 400-585 MW- off setting all new growth in air conditioning load over the next decade. If most of these TES installations are targeted for new construction or Transmission & Distribution (T&D) constrained areas, then TES could save over a Millions of Rupees of investment in the T&D system and perhaps equal savings in generation capacity investment.

Combining the operating costs savings with the capital cost savings means TES can help electricity suppliers significantly reduce its over-all costs. Since marginal fuel and capacity related costs are about 30-40 % of an electric utility’s total costs, reducing those by 30% for TES customers means the electric utility can save 9-12 % off its total costs in serving such customers. Certainly TES can be one tool in achieving such cost reductions.

  1. IMPLEMENTATION COST OF ICE THERMAL STORAGE SYSTEMS

Most TES systems cost more up front and (if cost-effective) pay off through reduced electricity bills. But carefully designed systems needn’t cost more— or much more—than conventional HVAC systems. Other factors that may help reduce cost of implementation include:

Smaller airflows will provide adequate cooling in new construction or major retrofits, especially for ice storage. This can decrease the duct diameter, reducing not only the cost of the ductwork, fans, and pumps but also the installation lab our cost. Smaller ducts may increase rentable space in new construction

System sizing is critical. Forex- ample, a careful assessment of the number of hours that peak load must be met with stored cooling could show that a considerably smaller storage system may only minimally affect demand reduction.

  1. SERVICE LIFE AND FIRST COST OF ICE THERMAL STORAGE SYSTEMS

Typically, a TES system increases costs compared to those for a direct cooling system. But a much larger picture needs to be looked at. Additional issues include

Electrical CapacityCan using TES reduce the capacity and therefore the cost of the electrical service to anew project, or avoid increasing the service in the case of a building expansion?

Useable Space:  Can use TES, with an underground storage tank, such as under a parking lot, free up space in an existing chiller plant or reduce the size of a new structure?

  1. OPERATION AND MAINTENECE REQUIREMENTS

Factors that tend to increase maintenance costs for cool storage systems compared to non-storage systems include:

Annual tests to ensure solutions contain proper coolant concentration, levels of corrosion inhibitors, and other additives for ice storage systems using glycol or other secondary coolants. Some glycol manufacturers provide free laboratory analysis of samples. There may also be an expense associated with replacing glycol lost during maintenance procedures and through leaks.

Increased water treatment expense in chilled water storage and some ice storage systems, which contain large volumes of chilled water in the tanks.

Factors that tend to decrease maintenance costs for cool storage systems include:

Smaller components, such as chillers, pumps, and cooling towers, for typical systems. Ants in the system provide coil freeze protection, and eliminate the need to drain the cooling system in the winter, or to use special controls to prevent coil freeze-ups.Cooling loads can be met from storage while some equipment is taken out of service for maintenance in some storage systems.

  1. REGULATIONS AND CODES COMPLIANCE REQUIREMENTS

All equipment and components used for a TES system should conform to the same laws, codes, and regulations required for traditional cooling systems. Large tanks may pose a problem. Zoning requirements, particularly height restrictions, should be checked early. If height is an issue, it is possible to completely or partially bury it. It is also possible that alevee will be required around the tank in case of a rupture.

If a fire-protection storage tank is required on site, as it would be at some manufacturing facilities, it may be possible to use this tank to store chilled water for the cooling system. If this is available, it should be carefully checked to ensure code compliance.

  1. CONCLUSIONS AND RECOMMENDATIONS

10.1  Total Energy Savings

TES provides major compelling benefits of concern to the Local power supply companies.

  • Energy efficiency(source and site)
  • Economic development (increased competitiveness of Country energy suppliers and energy users)
  • Deeming TES as a priority energy efficiency or Demand – Side Management program in Provinces energy resource policy decisions
  • Modify Building Standards to reflects TES’ source energy savings and peak demand reductions
  • Use TES as an air emissions control measure Country wide
  • Identify TES as a priority option for new and replacement cooling systems in “Competitive energy environmental partnerships “with key energy users, such as local and governmental buildings.

Based on the energy savings and other benefits of TES, several possible recommendations emerge for consideration. The first possible policy action is deeming TES as a priority energy efficiency or Demand- Side management programme in Pakistan energy resource policy decisions. TES has demonstrated significant energy and air emission savings and significantly improves load factor and provide cost savings that help both energy users and energy suppliers be more competitive.

The second possible policy action is to modify the Building Standards method of comparing alternative cooling technologies energy efficiencies; in particular, the method would reflect the source energy savings of TES.

10.2 Cooling thermal storage – Recommendations

There are valid reasons for the selection of ice storage system – reducing operating costs reducing the size of central chiller plant, reducing the building electrical maximum demand. The selection of the system on the grounds of energy and emissions savings is a fallacy. As far as environmental benefits are concerned, greater benefit would be obtained from publicising the improvements in conventional chiller design and control.

The ice storage is recommended for following circumstances with greater objectives.

Ice has a big advantage in smaller volume. This is insurmountable in situations where the building exists and space is at a premium.

11           REFERENCES                                         

  1. American Society of Heating Refrigeration, and Air-Conditioning Engineers, “ASHRAE Handbook HVAC Applications,” June
  2. 2. American Society of Heating Refrigeration, and Air-Conditioning Engineers, “Design Guide for Cool Thermal Storage,”
  3. 3. Electric Power Research Institute (EPRI) HVAC & R Center (formerly the Thermal Storage Air ConditioningCentre),UniversityofWisconsin,150EastGilmanStreet,Suite2200, Madison.
  4. International Thermal Storage Advisory Council (ITSAC), 3769 Eagle Street, San Diego, CA 92103, Tel: (619)295-6267.
  5. Donald Fiorino,(1992) “Thermal Storage programme for the 1990,s Energy Engineering” volume 89, Part 
  6. EPRI (1989) Electric Power Research Institute “Operation and performance of commercial cool storage system” 1989 volume 2: cu.6561
  7. ,(1990) Designing of thermal storage. Building services journal April 1990 pp.25
  8. (1995) Savings at Buildings society. Nation-wide control of frozen assets H&V Engineers,68 (726) 1995,pp.7,8-10
  9. , (1995) The Ice age dawns, Building services journal March 1995,pp 38.
  10. Salyer & Sircar.(1989) Development of PCM wall board for heating and cooling of Residential Buildings, University of Dayton Research Institute 1989,pp. 75-80
  11. Charles E. Dorgan & James S. Elleson , (1994a) Design guide for cool thermal storage 1994, Chapter No.3
  12. Charles E. Dorgan & James S. Elleson (1994b) Design guide for cool thermal storage 1994, Chapter No.3
  13. ITSAC (1996)) Inte’l Thermal storage advisory council, November 1996. Volume 11.
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Engineering Horizons

Engineering Horizons

“Engineering Horizons” is the first & leading technical magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics field under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.

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As you know, monthly “Engineering Horizons” is the first & Leading Technical Magazine of Pakistan covering Process, Mechanical, Metallurgical, Mining, Electrical & Electronics fields under a single cover. We also feel pleasure in saying that this is the only magazine of its own kind & style, which is widely circulated in all Engineering Sectors of Pakistan.
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