Energy recovery from LNG regasification for space cooling - IEEE Xplore

Energy recovery from LNG regasification for space cooling- technical and economic feasibility study for Singapore Ramanathan Subramanian

Bige Tunçer

PhD researcher, Architecture and Sustainable Design Pillar, Singapore University of Technology and Design, Singapore 487372. Email: [email protected]

Associate Professor, Architecture and Sustainable Design Pillar, Singapore University of Technology and Design, Singapore 487372. Email: [email protected]

Matthias Berger Chair of Information Architecture, ETH Zürich, Zurich 8093, Switzerland. Email: [email protected] Abstract—this paper investigates three scenarios for (LNG) Liquefied natural gas driven district cooling grids in terms of technological and economic feasibility. The scenarios consist of commercial, residential, and low temperature industrial cooling and their combinations. These scenarios achieve tremendous energy savings in comparison with conventional mechanical cooling. The free cooling potential in Singapore is calculated to Singapore Dollars (SGD) 800.75 million p.a. depending on the scenario, based on a regasification heat demand of 1009 kJ/kg of LNG with current terminal capacity of 3.5 Million Tonnes per annum (MTPA). In addition to the economic benefit, there is also an environmental benefit of 655.57 Million kg of CO2 e/ a (CO2 equivalent) through Greenhouse gas (GHG) emissions reduction by free cooling utilization. The terminal’s capacity has increased to 6 MTPA by the end of 2014. Index terms— District cooling, liquefied natural gas, regasification NOMENCLATURE BCA CBD CHP COP DHC EER EIA EPI GHG GIS HDB LNG NG ORV SGD SLNG TR URA

Building & Construction Authority Singapore. Central business district. Combined heat and power. Coefficient of performance. District heating and cooling. Energy efficiency ratio. U.S. Energy Information Administration. Energy Performance Index [kWh/m2/y] Green House Gas Emissions (kg of Co2 e/ a) Geographic information system. Housing and Development Board Singapore. Liquefied natural gas. Natural gas. Open rack vaporizer. Singapore Dollar. Singapore LNG Corporation Pte Ltd. Ton of refrigeration, 1 TR = 3.51685 kW (Kilowatt). Urban Redevelopment Authority Singapore.

978-1-5386-0971-2/17/$31.00 ©2017 IEEE

I. INTRODUCTION

S

INGAPORE is one of the major global oil hubs and since the late 1990’s invests heavily into NG, primarily for electricity generation [1]. LNG has become an economically feasible alternative to NG because of the need for a diversification of supply routes and reduced dependency on the existing pipelines from Malaysia. LNG is liquefied onshore or offshore by cooling it to cryogenic temperatures, and transported through tank ships to the LNG terminal in Singapore, this terminal opened in April 2014. LNG needs to be reheated and vaporized before end use. Heat from ocean water and additional gas burners during peak loads are currently employed for the regasification process of LNG to NG [2]. Dumping the available cooling this way creates an aquatic microclimate with adverse environmental impacts on marine life and is regarded as a harmful effect of LNG facilities [3-5]. Utilization of cooling from LNG regasification includes many applications and end uses like power generation, air separation into various components, refrigeration, cold storage, inlet air cooling for gas turbine power generation, production of dry ice and seawater desalination etc. Available cooling from LNG has been extensively utilized to almost 100% at the Senboku LNG terminals of Osaka Gas Co. Ltd., operating since 1978 in Osaka, Japan [6]. Most of the cooling applications have been in the terminal facility and only in 2011 utilization at a neighboring ethylene plant, which is slightly away from the terminal facility itself, started. Instead of considering the cooling available during the regasification of LNG as an inevitable side effect, it should be seen as an opportunity. Methods of utilizing the recovered cooling to enhance the efficiency of electricity production

through cryogenic CHP cycles have already been analyzed for technical and economic feasibility [7, 8]. Some suggestions include recovering available cooling for applications of direct cooling of close-by facilities [6, 9, 10] and also a few far off application for direct space cooling [11, 12]. Teracool [13] has developed a novel idea to reduce its data center’s cooling load and in parallel to generate electricity. Many recent studies post 2000’s have been conducted on LNG exergy recovery during regasification by utilizing the energy in running Rankine cycle, Choi et al.[14] proposed and investigated a cascade Rankine cycle that consisted of multiple stages of ORC and recovered LNG cooling for power generation. Sun et al. [15] proposed a Rankine cycle that utilizes a mix of three hydrocarbons as working fluid to utilize the available cooling in LNG effectively. Xue et al.[16] analyzed on a two-stage ORC with the low-grade heat of the exhaust flue gas of a gas-steam combined cycle powergenerating unit, as well as the cryogenic energy of LNG. Kim et al. [17]analyzed on a cascade power generation system utilizing cold exergy of LNG and adopted binary working fluids for each stage to minimize the exergy destroyed in the condensers of each stage of the cycle.

to reduce the carbon footprint by making the transition from oil to gas-fired power plants [20]. Between 1990’s and 2000 NG was used as a secondary, supporting fuel for electricity generation, with fuel oil being the dominant source, available more flexibly through ships instead of pipelines, and at a cheaper rate. In April 2014, Singapore’s first LNG terminal was inaugurated. This research was initiated immediately afterwards, looking into the demand for cooling as a business opportunity, by recovering the free energy for space cooling applications through district cooling grids. LNG to district cooling is a pragmatic application from an engineering perspective, however it is mostly overlooked by oil & gas companies as a beyond downstream or sidelined business opportunity to the mainstream business: selling uninterrupted NG supply for varied applications, predominantly electricity production. The opportunity of direct usage of LNG cooling as presented in Fig. 1 is an easier, cheaper and more convenient option compared to trigeneration [21, 22], as our study suggests. A general methodology for comparing and optimizing energy conversion with multiple carriers is presented in previous works [23, 24].

However, the spectrum of projects to utilize the recovered cooling for ranking cycle and energy production have been the maximum applications ventured out. However, near or at site is limited, most of the time due to availability of matching cooling demand and also due to safety and hazard requirements for isolating LNG terminals to less populated zones. In this paper we propose and evaluate two far sites and one local application in three scenarios with corresponding case studies, based on available heat (or reheating) demand from LNG regasification in Singapore. A commercial, a residential, and a low temperature industrial applications are simulated (in terms of heat and mass balances, thermodynamic principles) using the transient system simulation tool TRNSYS. Chilled water is generated from the available cooling recovered from the regasification process and compared to relying on mechanical air conditioning. The study includes a technical and economic feasibility analysis to evaluate the concept. II. LNG AND SINGAPORE CONTEXT Singapore has a tropical rainforest climate with an average daily mean temperature of about 27°C. Suffering from an urban heat island effect [18] the city of 5.4 million inhabitants has an ever-increasing need for cooling. To enable this cooling, Singapore is dependent on imported NG for 95% of its electricity mix [19]; The NG consumption of Singapore has more than quintupled from mid-1990’s until now, as part of a strategy

Fig. 1. Energy conversion chain for a) conventional electricity production from LNG, b) cogeneration, c) trigeneration, and d) cooling recovery.

Singapore’s experience with district cooling networks is well established in some large scale projects like the Marina Bay district cooling network with a capacity of 43’000 TR, and by projects from Keppel DHC with three plants and a combined plant capacity exceeding 60’000 TR [25, 26]. Singapore’s government has been keen in improving the overall effectiveness of the gasification and related processes, opening a grant call on “Competitive Grant Calls on Smart Grid and Gas Technology” in June 2014 [27]. III. LNG COOLING RECLAIM POTENTIAL Before evaluating the total cooling potential for

Singapore, first of all the specific heat required for heating LNG to ambient temperature levels has to be determined. Later we combine the specific heat with the total available amount of LNG: Singapore’s first LNG terminal by SLNG at the Jurong Island started with a capacity of 3.5 MTPA and has been increased to 6 MTPA by 2015 [28, 29]. The temperature and pressure of LNG stored in a vessel’s storage tanks is 111.15 K at 101.3 kPa. The LNG terminal is designed to deliver re-gasified NG at a maximum pressure of 4 MPa and a minimum temperature of 286.15 K [30]. Large amounts of mechanical energy are required for the liquefaction of NG to LNG, estimated to be 0.850 kWh/kg of LNG [31]. For a typical LNG composition presented by Hizasumi [32], the available chemical energy of LNG is estimated to be 14.4 kWh/ kg of LNG, whereas the corresponding available cooling of LNG is approximately 0.240 kWh/kg of LNG, less than 2% of chemical energy. The latter partially explains the disinterest of oil & gas companies in this matter. Since LNG coming into SLNG’s terminal is from varied sources, data on the exact composition of the LNG mix was not available. For simplification we have assumed the LNG to be of 100% methane (CH4). By applying the idea of exergy analysis [33], the low temperature exergy Ex,th can be defined as the thermal nonequilibrium exergy with the environment under the system pressure.

E x, th = E x ( P, T ) − E x ( P , T0 ) When LNG is heated from the low temperature to ambient temperature, a boiling phase change and sensible heating will occur consecutively. Suppose the average phase temperature is Ts, and the vaporization latent heat exergy that LNG absorbed is

(T0

TS ) ⋅ r , where r is the latent heat of

evaporation. The apparent heat exergy that low temperature natural gas absorbed from Ts to ambient temperature T0 is given

as

³

Ts

T0

T · § C p ¨ 1 − 0 ¸ dT T ¹ ©

and

we

As per the configuration of the closed loop heat exchanger network for cold reclamation, the boundary conditions are given as T0 = 278.15 K (Temp at which NG is supplied to town gas and other applications), Ts = 111.67 K (Phase change temperature of LNG to NG). Before end use NG is compressed for pipeline transport to the NG-fired power plant, from 278.15 K at 101.3 kPa to 286.15 K at 4 MPa. The latent heat of vaporization is r = 510.83 kJ/kg, and the specific heat capacity is cp = 2.148 kJ/kg/K (liquid phase). Thus we calculate the Ex,th = 1009 kJ/kg. Szargut et al, [34] worked out the thermal exergy 724 kJ/kg for a mix of LNG with LNG temperature limits of 276.15 K and 139.15 K for exergy extraction. Overall estimates of the cooling potential as per LNG terminal’s current capacity of 3.5 MTPA and 1009 kJ/kg of LNG is 980 GWh/a thermal cooling load, and equates to 223.20 GWh/a electric energy to meet the thermal cooling loads. This corresponds to annual savings of SGD 54.7 million, considering commercial electricity cost of 24.51 cents/kWh [35], chiller’s efficiency of 0.80 kW/RT refrigeration [36], the COP = 4.39 and EER=15. IV. SYSTEM DESIGN FOR LNG TO DISTRICT COOLING GRID In order to reclaim the availabe cooling during the LNG regasification process, it is necessary to construct a closed heat exchange network using a suitable secondary refrigerant. As per the operating temperature requirement for low temperature cooling as well, water + glycol mixture with 80% of glycol is selected, with a freezing temperature below 222 K [37]. All elements of the network are designed to operate at atmospheric pressure of 101.3 kPa. The heat exchanging process relies on two heat exchangers in closed loop, as in Fig. 2. The primary exchanger handles the cold LNG on the source side and the secondary refrigerant (water + glycol mixture) on load side. The temperature profiles over the heat exchangers are shown below as per Table 1.

are

concentrating mainly only on the thermal non-equilibrium exergy and so the pressure nonequilibrium exergy amounting to a smaller fraction is assumed negligible, with the assumption of maintaining system pressure to be almost constant.

Thus, the total low temperature exergy as the sum of the vaporization latent heat exergy and the apparent heat exergy is Ts § T · §T · E x ,th = ¨ 0 − 1¸ ∗ r + ³ C p ¨1 − 0 ¸ dT T 0 T ¹ © © Ts ¹

Fig. 2.

Circuit diagram of closed loop exchanger network in TRNSYS.

The secondary exchanger has the water + glycol mixture on source side and chilled water as per load requirements. The generic closed loop heat exchanger network configuration, in order to cater for the three scenarios of cooling applications, is designed as shown in Fig. 2. The

configuration remains the same for commercial, residential scenarios and the combined scenario (Low temp + Residential), but the circuit’s complexity reduces for low temperature cooling application as the water + glycol mixture is directly used as the refrigerant to cool down the space to cryogenic temperatures (see Tables 1 and 2). For the low temperature cryogenic cooling application (i.e., cold warehouse).

The actual amount of NG imported as LNG was considered to be 3.5 Million Tons per annum, the current capacity of the LNG terminal since May 2013 [40].

Table 1. List of operating temperatures in the three scenarios.

Chilled Water Temperature in [K] Scenario

Supply

Return

ǻT

Commercial

279.15

289.15

10

Residential

279.15

289.15

10

Low Temp. Warehouse

228.15

303.15

75

Low Temp (Combined Case)

228.15

288.15

60

Residential (Combined Case)

279.15

289.15

10

Table 2. Thermal properties of the refrigerants.

Refrigerant

Specific heat capacity in [kJ/kg·K]

LNG (Methane)

2.148

20% Water + 80% Glycol

2.845

Chilled water

4.186

Since the demand and supply profiles of available cool energy do not match, there is a need to account for thermal storages to balance the heat and mass transfer on LNG and water + glycol mixture side for the primary exchanger as well as for the water + glycol mixture and chilled water flow for the secondary heat exchanger. For both cases, a variable volume tank (TRNSYS Thermal Storage TYPE -39) with initial volume to meet the minimum mass flow demand is assumed to balance heat transfer through the exchangers (see Fig. 2). A detailed sizing of the storage tank, as per the simulation results are highlighted later for the various scenarios. V. LOAD CURVES TO FLOW RATES - LNG SUPPLY AND COOLING DEMAND A. LNG supply The load curve for LNG supply was not directly available. Since LNG cannot be stored in large quantities locally, it basically fuels the nearby electricity plants directly for the time the LNG tanker debarks. Hence, we obtained a load curve through the electricity load pattern for a typical day in Singapore (January 2011) [38] with a peak demand of 6 GW. Data on the Singapore fuel mix from EIA [39] for 2011 was used to ascertain the contribution of NG to produce electricity. Singapore’s natural gas balance statistics from 2009 to 2011 [40] were used to calculate the exact amount of NG out of the electricity generation statistics. The electricity demand was then converted into the actual flow rate of LNG.

Fig. 3.

End use demand to chilled water flow conversion (Source: [34]).

B. Cooling demand The commercial scenario consists of several usage types: hotels, shopping malls, hospital and offices as presented in Fig. 3. The Residential scenario includes HDB-style housing (HDB is public housing, designed and developed by the Singapore government) and detached houses. The low temperature applications include power plant cooling for excess cold, but mainly the cold storage warehouses as bulk consumer. The load statistics for the various end use applications were obtained through the works of Quah in 2010 [41]. The average energy consumption statistics for building typologies was obtained through the EUI in kWh/m2·a, same as EPI [42]. The electricity demand for space cooling was then converted into thermal demand by considering a chiller efficiency of 0.8kW/t. The cooling demand for the given area in m2 was converted into the flow rate of chilled water (required in kg/h): Thermal Cooling Load required (in kW) = (Fraction_Demand · 0.002738 · Cooling_Percent · EUI · Chiller_Efficiency · Area_m2 · 41.66) / 1000

Where, Fraction_Demand is the demand Fraction of the load from existing load pattern/ profile for the corresponding typology of the scenario, Cooling_Percent is the percentage of the overall load used for space cooling application [in %], EUI is the Energy Utilization Index [kWh/ (m2.y)], Chiller_Efficiency is the Efficacy of the Chiller used to convert the Electrical load into thermal load [in kW/Ton], and Area_m2 is the overall built-up area of the building typology as per the respective scenario [in m2] Mass flow rate of chilled water in [kg/h] = Thermal Cooling Load Required (in kW) · 3600 / (Specific Heat of Chilled Water · (Temp_Return – Temp_Supply)).

Site context district cooling- Jurong Lake District, Singapore

Fig. 5. Fig. 4. Jurong Gateway map and current developments. (Image from IDA, Smart & Connected Jurong Lake District Pilots & Trials, Call for Collaboration, Public Document Version 1, 28 March 2013)

The area identified for the district cooling case study is the Jurong Lake District of Singapore with the Jurong Gateway development in the Eastern part, as presented in Fig. 4. Other parts in the West include the lake itself, residential areas, hotels, a golf course and some unplanned areas. This district is designated as a new mixed-use town center by URA [43], and is currently under development. The 70ha Jurong Gateway commercial precinct will provide about 500,000 m2 of office space for businesses that do not require a CBD location, 250,000 m2 for retail, food & beverage, entertainment uses to support office workers and nearby residents, and at least 1000 private dwelling units. About 30% of the offices, 80% of the retail spaces, 70% of the residential units and 10% of the hotel rooms planned in the precinct are currently at different stages of construction and will be completed progressively from 2014 to 2016. Buildings are also required to achieve the BCA Green Mark Gold plus rating as part of sustainable development initiative. Fig. 5 presents a GIS image of the intended pipe routing including the overall pipe length. This information was used in the energy model to calculate the heat loss through pipes, for both supply and return pipelines. The location of the LNG terminal is along the coast of Singapore at point ‘A’ in Fig. 5, in one of the industrial and petrochemical corridors of Jurong Island. The choice of the location is strategic to enable the tanker ships to unload the LNG with ease to the onshore storage tanks. The distance of the Jurong Gateway development precinct at point ‘B’ in Fig. 5 is 17.1 km from the LNG terminal. Knowing the distance and routing is critical to optimize the pipe dimensioning to reduce the heat losses from the pipe to the environment versus friction losses and other parameters (material and geometry).

Distance and routing of LNG terminal to site.

VI. SIMULATION ENERGY PERFORMANCE STATISTICS Knowing the available cooling, the load curve and the routing, the next step is simulating the dynamic behavior of the system. The closed loop heat exchanger network is represented as a dynamic energy simulation model for all three individual scenarios separately and one combined scenario (Low Temp+ Residential) in TRNSYS. The model is initially tweaked for equipment sizing and for heat and mass transfer balances. The load characteristics, temperature profiles of LNG, secondary refrigerant and chilled water are analyzed and plotted for a week. The required pumping power, for all the three individual pumps, namely LNG pump, secondary Refrigerant Pump and chilled water pump, are calculated considering the pump head loss due to friction. This helps to estimate the operational energy required to enable the cool energy reclamation from LNG regasification to chilled water generation. A. Commercial scenario Table 3 shows the parameters that define the space cooling energy requirement as defined before in section V: the area, EPI [42], percentage of cooling energy to total energy, and demand of space. The area for the various commercial uses / functions has been obtained from various future revamp plans [44] and existing shopping mall information [45, 46]. Table 3. Parameters for commercial scenario load calculations

Commercial use Function

Unit 2

Office

Hotel

Retail

Hospital

Area

m

800,000

190,000

691,178

169,000

EPI

kWh/m2/a

351

230

145

230

Cooling load

%

60

50

50

40

Electricity load

GWh/a

168.48

21.85

50.11

Total area

m2

1,850,178

Total elect. load

GWh/a

255.90

Chiller eff.

kW/t

0.8

Total cool. load

GWh/a

1125

15.55

The overall cooling demand for the commercial space of 1.85 Million m2 is estimated to be 1125 GWh/a. The maximum possible cooling energy reclaimed from LNG regasification process is 980 GWh/a, so LNG cooling can offset only 87.19% of total commercial space cooling demand. Fig. 7 presents the demand for chilled water (in [kg/h]) for the various functions and spaces such as office, hotel, retail and hospital in a weekly profile. The role of the thermal storage is shown in Fig. 6. The excess cooling carried away by secondary refrigerant is stored in the variable storage and used, when required by the demand for space cooling. As can be seen in Fig. 7, the demand for water + glycol mixture to transfer cooling to Chilled water overshoots during mid-day till evening for space cooling as per the load statistics, the supply of glycol + water secondary refrigerant from storage is as per the load profile of commercial demand, as per Fig.7.

Fig. 6. Need for thermal storage due to demand and supply load mismatch in the commercial scenario.

Commercial

0.873

111.15

278.15

303.15

243.15

Residential

0.873

111.15

278.15

303.15

243.15

Low Temp

0.8697

111.15

278.15

303.15

228.15

Low Temp+ Residential

0.8697

111.15

278.15

303.15

228.15

Scenario

Secondary Heat exchanger effectiveness

TEMPERATURE PROFILES (SECONDARY EXCHANGER) IN K Water + Glycol IN

Water + Glycol OUT

CHW IN

CHW OUT

Commercial

0.543

243.15

268.15

289.15

279.15

Residential

0.543

243.15

268.15

289.15

279.15

Low Temp

NA

228.15

303.15

NIL

NIL

Low Temp+ Residential

0.9836

228.15

288.15

289.15

279.15

Table 5. Temperature profiles and parameters of Secondary Heat Exchanger

Fig. 7.

Load Curve for the commercial scenario.

Table 4. Temperature profiles and parameters of Primary Heat Exchanger

Scenario

Primary Heat exchanger effectiveness

TEMPERATURE PROFILES (PRIMARY EXCHANGER) IN K

LNG IN

LNG OUT

Water + Glycol IN

Water + Glycol OUT

The overall pumping power for all the three pumps, that make up the closed loop heat exchange network was obtained as an output from the energy model simulation and was found to be 3.51 GWh/a and SGD 0.86 million/a considering electricity cost of SGD 24.51 cents/ kWh. 87.19% of total commercial space cooling is offset by LNG regasification cooling reclamation and a total economic

Fig.8. Air-conditioning and space cooling load of total energy demand (Source: EMA and SP services, Sept 2009).

savings (or earnings) are SGD 61.88 million SGD/a after subtracting the network costs. Refer to Table 4 and Table 5 for the heat exchanger effectiveness and also the temperature profiles of the various refrigerants in the closed loop circuits across the two exchangers. The glycol + water mix (secondary refrigerant)

has a ǻT of 25 K over the secondary exchanger and the exit temperature of 268.15 K is elevated to 303.15 through ground source heat pumps (pumping the secondary refrigerant through pipes at 10m soil depth). Thus the temperature of return to the primary storage would be 303.15 K. The remaining 12.81% commercial space, which could not be serviced by the LNG exergy cooling could be served by a conventional mechanical cooling system, operated by electricity. As the LNG terminal’s capacity is expected to increase to 6 MTPA by end of 2014, this load could also be met at mere expense of recurring pumping and maintenance cost and reducing the need for electricity to generate equivalent mechanical cooling. B. Residential scenario The parameters that define the space cooling energy requirement are presented in Table 6, as defined before in section V. The total area of HDB blocks in Jurong East was taken from HDB statistics [47], the EPI index is 145 kWh/m2·a and the cooling load percentage is set to 33% [42, 47] (see Fig. 9). We then convert this kWh/a cooling load into a flow rate of chilled water, as required by the cooling demand. Table 6. Parameters for residential scenario load calculations

maximum possible cooling energy reclaimed from LNG regasification process is 980 GWh/a, therefore LNG cooling can offset only 100% of total residential space cooling demand and have 52.17% of free cooling as unused end use application. The temperature profiles are similar to the commercial case since the primary heat exchanger circuit remains the same, including the set-point temperatures and heat exchanger effectiveness. The only change is the flow profile from the storage to load and recharge/discharge frequency of the storage. Fig. 9 presents the flowrate of chilled water required as per the fractional load profile for a 3-room HDB flat. The overall annual chilled water flow rate was estimated as 11.298 million liters similar to the commercial scenario, but the energy intensity of the residential scenario is much lower at 145 kWh/m2·a compared to 351 kWh/m2·a for office space (dominant area of commercial scenario) [42]. Fig. 10 presents the charging and discharge of a thermal storage by glycol + water mix , for matching the load profiles on supply and demand side. As total residential space cooling demand is offset by LNG regasification cooling reclamation and total economic savings of SGD 26.13 million/a (energy cost savings) are feasible. The pumping cost is similar to the commercial scenario. The total savings are SGD 25.27 million SGD/a after subtracting the pumping load cost 0.86 million SGD.

Type of dwelling Function

Units

1room

2room

3room

4room

Number of units

No unit

352

412

6821

7855

Area

m2

31

45

68

Executive Studio 5room

96

HUDC

Number of units

No unit

5925

1871

143

0

Area

m2

118

145

40

156

Total area

m2

2,228,514

EPI

kWh/m2/a

145

Cooling load

%

33

Total elect. load

GWh/a

106.63

Chiller eff.

kW/t

0.8

Total cool. load

GWh/a

468.65

Since there is limited firsthand information on the detached houses in the Jurong precinct and about 81.9% of Singapore’s resident population lives in HDB [47], we have not considered the detached houses for estimating the loads. The overall cooling demand for the residential space of 2.22 million m2 is estimated to be 468.65 GWh/a. The

Fig. 9.

Load Curve for residential scenario.

Fig. 10. Need for thermal storage due to demand and supply load mismatch in the residential scenario.

C. Low temperature cold storage/ warehouse scenario The low temperature scenario was conceived to use the cryogenic cooling available from LNG to be directly used for space cooling of applications such as warehouse or cold storage demanding very low temperatures in the -50°C to 20°C range. A water + glycol mixture is again required to transport the available cooling directly for space cooling at

very low temperatures. The complexity of the heat exchanger circuit is reduced by the omission of a secondary heat exchanger. The load profile, for such a low temperature warehouse cooling maintained at -45°C (set-point) is assumed to be constant throughout the year. All the available 980 GWh/a of thermal cooling from LNG regasification is utilized to generate chilled water supplied at -45°C and return at 30°C with a temperature difference of 75 K (ǻT= 75 K). A thermal storage is also utilized here for the buffer required to store the cooling, when under-utilized by the demand profile and vice versa. Fig. 11 presents the chilled water need and the available cooling from the secondary coolant. The demand is assumed to be constant, and this helps to further justify the need for a thermal storage. The annual chilled water demand is estimated as 43.15 million liters.

Fig. 11. Need for thermal storage due to demand and supply load mismatch in the low temperature scenario.

The energy incurred in producing 980 GWh/a of thermal cooling load at very low temperatures (-50°C to -20°C) assuming a COP of 0.3 and a chiller efficiency of 11.72 kW/t is estimated as 3269.90 GWh/a. At SGD 0.2451/kWh cost, the overall cost of meeting the cooling load is SGD 801.45 million/a. The pumping energy cost is estimated by the energy model to be SGD 1.38 million. The overall savings from using the available free cooling for cryogenic cooling application amounts to SGD 800.07 million/a. Assuming an EPI index 800 kWh/m2·a for a typical warehouse facility, we can use the free cooling to provide cryogenic cooling at 35°C for an area of 1.22 million m2.

As per Fig.12, the load in terms of Chilled water requirement are shown, it is interesting the note the assumption of the low temperature scenario is, with a constant load throughout and the residential scenario fluctuates as per the hourly demand. Fig.13 shows the need for the thermal storage and also the charging and discharging cycles of the storage for glycol + water mixture, to match the combined load of Low temp and residential applications. Table 7, highlights the frictional loss or pump head in m for the various scenarios as per the flow rate, pump length and the material finish of the pipe interior. This values have been included to calculate the pumping power for the various pumps, to account for the overall pumping load for each scenario in pumping the various liquids LNG, glycol + water mix and chilled water. The total pump efficiency of 0.6 and motor efficiency of 0.9 were used to calculation of the pumping power. Table 8, highlights the maximum storage capacities drawn from the simulation of various cases. This has helped us to size the dimensions for the storage tanks, which were initially assumed to be of infinite variable storage with TRNSYS. This could further help us to reduce the thermal losses, but effective sizing of the storage systems. A possible sizing with a cylindrical storage scheme is highlighted in the table. The storage needs to be well insulated in order to maintain the temperature profiles with less losses. The economic valuation of thermal storage was not carried out separately, due to lack of expertise and availability of more data on storage systems/ unit volume of energy storage. The overall energy from the combined case of Low Temp and Residential scenario mounted to 1816.62 GWh/a. (account for 11.72 kW/ton chiller efficiency for low temp case and 0.8 kW/ton for residential case). SGD 0.2451/kWh cost, the overall cost of meeting the cooling load is SGD 445.22 million/a, account for 0.76 Million SGD pumping demand.

D. Combined Case: Low temperature + Residential Scenario The combined scenario of linking Low temperature and Residential scenario, is because of the low demand for residential case and so, operating it stand alone would require a larger storage to store the rest of unused cooling energy extracted from LNG (52.3% unused LNG cooling). So we can combine this case to include 100% residential cooling load of 468.65 GWh/a accounting for 2.22 million m2 residential area and rest 52.3% unused cooling could be used to cater for an area of 0.64 million m2 of low temperature cooling (assuming an EPI of 800 kWh/m2·a for a typical warehouse facility).

Fig. 12. Load Curve for Low Temp + Residential combined scenario.

Low_Temp

1800.00

6

10

Low_Temp+Residential

2663.22

8

10

VII. ECONOMIC FEASIBILITY STUDY The overall closed loop heat exchanger network is designed with the equipment as presented in Fig. 3. Due to the unavailability of cost details for most of the equipment, an extensive economic analysis to ascertain the investment capital cost for setting up the network was not established. Fig. 13 Need for thermal storage due to demand and supply load mismatch in the low temperature + residential scenario.

Table 7: Frictional loss calculations of various scenarios for pumping load calculations

Scenario

Commercial

Residential

Low_Temp

Low_Temp+Residential

Pipe Component

Frictional Loss in Pipe or Total Dynamic Head (m)

LNG Pipe

193.75

Water+ Glycol (80%) (Primary)

7.25

Water+ Glycol (80%) (Secondary)

318.98

Chilled Water (Commercial)

124.33

LNG Pipe

193.75


7.25


63.36


24.62

LNG Pipe

193.75


32.53


NA


NA

LNG Pipe

193.75


4.80


9.79


24.62

To get a comparative scale of investment cost required, we reference the Marina Bay Sands district cooling system in Singapore, which is operational since 2006. The original planning envisaged a service area with over 8,000,000 m2 in gross floor area. The overall chilled water pipe network length was 5 km in May, 2010 [48]. As on September 2011, phases 1 and 2 of the district cooling system are in operation, providing 1.1 million sq. m of accommodation with cool air via two chilled water production plant [49]. The overall investment cost incurred for the 5 km pipeline was in two phases of 3 km; phase I costing SGD 81 million and phase II costing SGD 137 million and the equipment cost was indicated to be SGD 110 million [50]. No statistics are available on the operating cost of the Marina Bay district cooling system, making it difficult to understand the economics behind like payback, breakeven and fuel specific cost inflations over the lifetime of the equipment, and additionally the deteriorating efficiency of the equipment/plant. It would be possible to visualize the investment cost going into building this district cooling network over a length of 34.2 km of pipeline from Jurong Island to the Jurong Gateway precinct. The main difference here is that the Marina Bay district cooling system works with electricity as the source of power to drive the chiller plants. In our LNGregasification free cooling reclamation, we negate the need for fuel to power any system, and our chilled water is directly available, consequently reducing losses as no conversion from electricity to cooling and vice versa occurs. Thus the driving factors that differentiate conventional fuel/ electricity powered district cooling and our approach of using reclaimed cooling are the need for a comparative load scenario, high investment cost, but less operational cost and reduced dependency on fuel and its inflating costs.

Table 8: Maximum storage size calculation for cylindrical thermal storage

Scenario

Max capacity of storage (m3)

Height (m)

Radius (m)

Commercial

12289.56

10

20

Residential

12289.56

10

20

Having access to more comparative values would help us understand the payback period and reduced investment cost as the demand increases over and above a threshold, to maximum utilize the available free cooling. Since, there exists a similarity in cooling capacity of space cooling 1.1 million sq. m for the Marina bay District cooling system (Phase1 and 2, for which we have statistics) and our low temperature cold storage/ warehouse scenario, cooling demand estimate of 1.44 million sq. m, we can generate an

approximate cost estimate with some assumptions as stated below. •

Cost of Equipment (Assuming cost similarity for High efficiency Heat exchangers and pumps today with respect to absorption chiller and pumps in 2006) and also assuming 25% money value inflation = SGD 110 * 1.25 = 137.5 Million

•

Cost of Infrastructure (Piping and other infrastructural costs) for 5 km (2011), Money inflation 25% = 218 Million SGD. The high piping infrastructure costing is due to the unique Common service tunnel and district cooling system network built up at 200 tons of concrete for 1.4 km of the project at a cost of 51 Million USD [51]

•

Considering an appreciation in cost of 50% towards improved insulation of piping (for cryogenic temps), inclusive of 25% money inflation and accounting for 34.4 km pipe length = 218 * 1.50 *34.4/5 = 2249 Million SGD

•

Operations cost is a meager 1.79 Million SGD/a, for pumping the fluids through the series of heat exchangers (is negligible when compare to the overall investment cost and savings potential over the year). Maintenance cost at 2% of project value = 47.73 Million SGD

•

Net Investment cost excluding maintenance cost = 2387.26 Million SGD and Net savings from free cooling/a = 445.75 Million SGD/a (as per section VI, case D Combined Case: Low temperature + Residential Scenario)

•

Calculating the Payback period (Excluding maintenance) = 2387.26 Million SGD/ 445.75 Million SGD/a = 5.33 Years approx.

Assuming an average lifetime of 10 years for the heat exchanger and network system, we can make a profit of = 2228 Million SGD for the time period from 5-10 years @ the expense of no actual fuel or electricity and with just by pumping water through the network for an energy intense cryogenic application with a EPI of 800 kWh/m2·a. Also the risk of fuel dependency is reduced. This is just a typical figure as per the current LNG import statistics and could grow a greater extent. As the current LNG import as per 2014 statistics stands at 11%. Also, the savings is calculated as per the electrical energy required for offsetting the thermal space cooling demand, this rate of 24.51 cent/kWh could also increase in the future predicted for increased fuel prices. Thus the savings here is just a conservative one and could well be more, which improves the payback further and supporting towards the inclusion of

this high yielding project opportunity towards the unnoticed or less cared lower stream LNG process chain. There is also a saving of 361 kg Co2 e/ Mwh [52] for producing 1 Mwh of electricity and so for the amount of savings that we have i.e. 1816 Gwh/a of electricity requirement. (Conservative combined case of Low temp + Residential scenario) So, the overall savings in GHG emissions possible is 655.57 Million kg of Co2 e/.a This approach on LNG cooling reuse, is thus an economically beneficial and environmentally sustainable, with least possible investment and reduced dependency on fuel as a source of cooling, which would have otherwise been the case. This project could thus help Singapore’s audacious pledge to stabilize greenhouse gas emissions by 2030, by reducing emissions intensity by 36 per cent from 2005 [53]

VIII. CONCLUSION The scope of this paper is to evaluate the overall efficiency of a potential LNG beyond-downstream business model, by efficient reclamation of available cooling and its application in three scenarios. More accurate results can be obtained by using data on the LNG mix from the terminal, load statistics that are more detailed, and cost parameters for the equipment and infrastructure. The overall savings potential from using the available free cooling for space cooling in various scenarios are summarized in Table 5. The savings potential is maximum for the very low temperature (or cryogenic) cooling scenario. Being the most energy intensive process to produce subzero temperatures as indicated by the high energy consumption rather than produce chilled water at 6°C has a strong leverage effect on the costs. Hence the potential savings are indirect proportional to the cooling temperature in the application. Table 5. Energy savings potential from various scenarios compared to available LNG cooling at energy costs of SGD 24.51 cent/kWh.*:Savings before network costs assuming to cover 100% load share.

Evaluated scenario Function Available Commercial Residential

Low temp.

Low temp + Residential

Total cool. load in [GWh/a]

980

1125.1

468.7

980

980

Chiller eff. in [kW/t]

0.8

0.8

0.8

11.72

11.72 & 0.8

Tot. elect. load in [GWh/a]

223.0

256.0

106.6

3269.9

1819.9

Total savings* in [mill. SGD]

54.706

62.7

26.1

801.45

Load share in [%]

-

87.13

100

100

445.25

52.3 and 47.7

Network costs in [mill. SGD]

[18] -

0.86

0.86

1.38

0.76 [19]

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