Utafiti huu unachunguza jinsi mabadiliko ya kila saa katika uzalishaji wa upepo na jua yanavyobadilisha utendaji halisi wa mfumo wa pumped thermal electricity storage unaotumia Brayton cycle. Mfumo uliotathminiwa huhifadhi ziada ya umeme kupitia CO₂-based heat-pump cycle katika molten salt ya joto la juu na katika ethylene glycol solution ya joto la chini; upungufu wa umeme unapotokea, hubadilisha joto lililohifadhiwa kurudi kuwa umeme kupitia Brayton heat-engine cycle.

Researchers walijenga modeli ya 8.760-hour quasi-dynamic operation inayounganisha 300 MW wind na 300 MW photovoltaic generation zinazowakilisha eneo la Qinghai nchini China, local consumption yenye 70 MW peak load, 80 MW external-grid transfer limit, state of charge ya storage tank, na part-load losses za compressor na expander. Reference system ya awali ina 100 MW discharge power na six-hour storage duration.

Katika fixed design-point approach, annual discharged electricity ilihesabiwa kuwa 219,00 GWh, round-trip efficiency %66,92 na levelized cost of storage 0,1365 USD/kWh. Real-time chronological operation, SOC limits na part-load degradation zilipozingatiwa, annual discharge ilishuka hadi 94,20 GWh na dynamic round-trip efficiency hadi %55,81; LCOS ikapanda hadi 0,2753 USD/kWh. Hivyo static assessment katika scenario hii ilionyesha realistic cost ikiwa lower kwa approximately %101,67.

Power na energy capacity zilipooptimized together, most economical wind-solar hybrid configuration ilipatikana kwa power scale sawa na %30 ya reference power na 12-hour storage duration. Katika configuration hii, charge na discharge powers zilikuwa respectively approximately 44,8 MW na 30,0 MW, dynamic efficiency %64,37 na LCOS 0,1772 USD/kWh. Results zinaonyesha kuwa kujenga system kubwa ili kunasa highest instantaneous renewable-energy surplus si kila mara most economical solution.

Maelezo ya Kina

Tatizo kuu la utafiti ni nini?

Wind na solar power generation hubadilika kila mara kulingana na saa ya siku, weather conditions na season. Local electricity demand haifuati temporal pattern ileile. Renewable generation ikizidi total power inayoweza kutumiwa na local load pamoja na power inayoweza kuhamishwa kwenda external grid, sehemu ya usable energy hucurtailed au kukatwa. Katika saa ambazo generation iko chini ya local demand, electricity deficit hutokea.

Energy storage system inaweza kuhamisha energy kati ya periods hizi mbili. Hata hivyo, kuangalia only nominal power, storage duration na design-point efficiency ya storage haitoshi. Economic na thermodynamic performance ya system huathiriwa kwa pamoja na conditions zifuatazo:

• Renewable-energy surplus hutokea katika saa zipi na kwa power levels zipi,
• Ukubwa na duration ya local electricity deficit,
• Kama storage tank imejaa au iko empty wakati huo,
• Charge na discharge equipment zinafanya kazi kwa load gani relative to nominal power,
• Compressor na expander hupoteza efficiency kiasi gani katika part load,
• Installed capacity inaweza kutumika kwa kiwango gani katika mwaka mzima.

Main research question ni hii: PTES system iliyounganishwa na wind-solar microgrid, inapofanya kazi mwaka mzima under real chronological constraints, inatofautiana kwa kiasi gani na design-point calculations, na matching ya power na storage duration inapaswa kubadilishwaje?

Pumped thermal electricity storage ni nini?

Pumped thermal electricity storage au PTES ni long-duration storage technology inayobadilisha electricity kwanza kuwa hot na cold energy, kisha kurudisha energy hiyo kuwa electricity. Badala ya kuhifadhi energy kwa electrochemical form kama batteries, hutumia temperature difference kati ya hot na cold storage media.

System iliyochunguzwa katika study hii inategemea closed CO₂ Brayton cycle. High-temperature storage medium ni four-component molten salt yenye broad liquid temperature range, huku low-temperature storage medium ikiwa ethylene glycol-water solution.

Charge cycle

Figure 1(a) inaonyesha charge cycle inayotokea wakati wind na solar generation zinazidi power ambayo microgrid inaweza kukubali:

1. Electricity yenye curtailment risk huendesha compressor kupitia motor.
2. CO₂ hubanwa hadi high pressure na temperature.
3. CO₂ huhamisha heat kwenda molten salt katika high-temperature heat exchanger.
4. Regenerator hurecover sehemu ya heat ndani ya cycle.
5. Cold energy huhifadhiwa kupitia expander na low-temperature heat exchanger.
6. Electrical energy huhifadhiwa katika form ya hot molten salt na cooled ethylene glycol solution.

Wakati wa charge, hot-storage side hutumia high-temperature hot tank na low-temperature hot tank; cold-storage side hutumia high-temperature cold tank na low-temperature cold tank.

Discharge cycle

Figure 1(b) inaonyesha discharge cycle inayotokea wakati local electricity demand inazidi wind-solar generation:

1. CO₂ kwanza hubanwa katika compressor.
2. Hupokea heat kutoka hot molten salt kupitia regenerator na high-temperature heat exchanger.
3. High-temperature CO₂ hupita kwenye expander na kuzalisha mechanical power.
4. Expander huendesha generator na kuzalisha electricity.
5. CO₂ hukamilisha cycle kupitia air cooler na low-temperature heat exchanger.

Hivyo renewable-energy surplus inaweza kuhamishwa kwenda saa nyingine na kukidhi sehemu ya local load deficit.

Kwa nini system inachukuliwa kuwa flexible kijiografia?

PTES haihitaji lazima large elevation differences kama pumped-storage hydropower au special underground geology kama compressed-air storage. Capacities za power-conversion equipment na thermal-storage tanks zinaweza kuchaguliwa separately kwa kiwango fulani. Feature hii inaruhusu different power na storage-duration combinations kujengwa.

Hata hivyo, PTES si storage tanks pekee. Compressor, expander, motor, generator, heat exchangers, regenerator, air cooler na circulation equipment lazima zifanye kazi pamoja. Kwa hiyo low utilization ratio na part-load operation vinaweza kuathiri system cost kwa kiasi kikubwa.

Basic design values za reference system ni zipi?

Kwa nini molten salt properties ni muhimu?

Storage tank volume na heat-exchanger design hutegemea density, specific heat capacity, viscosity na usable temperature range ya molten salt. Temperature ya salt inapoongezeka kutoka 200 °C hadi 650 °C katika study, density hupungua kutoka approximately 2.078 kg/m³ hadi 1.604 kg/m³; viscosity hupungua kutoka 10,4 mPa·s hadi 1,26 mPa·s.

Specific heat capacity hubadilika nonlinearly na temperature na imetolewa kuwa 0,509 kJ/(kg·K) at 200 °C na 1,706 kJ/(kg·K) at 650 °C. Model hutumia temperature-dependent properties hizi katika thermodynamic calculations.

Basic thermodynamic equations ni zipi?

Compressor

Isentropic efficiency ya compressor:

\[ \eta_{\mathrm{com}} = \frac{h_{\mathrm{com,out,is}}-h_{\mathrm{com,in}}} {h_{\mathrm{com,out}}-h_{\mathrm{com,in}}} \]

• η_com: compressor isentropic efficiency,
• h_com,in: compressor inlet specific enthalpy, kJ/kg,
• h_com,out,is: isentropic outlet specific enthalpy, kJ/kg,
• h_com,out: actual outlet specific enthalpy, kJ/kg.

Compressor power:

\[ W_{\mathrm{com}} = \dot{m}_{\mathrm{com}} \left( h_{\mathrm{com,out}}-h_{\mathrm{com,in}} \right) \]

Hapa ṁ_com ni CO₂ mass flow rate na W_com ni power consumed by compressor.

Expander

Isentropic efficiency ya expander:

\[ \eta_{\mathrm{exp}} = \frac{h_{\mathrm{exp,in}}-h_{\mathrm{exp,out}}} {h_{\mathrm{exp,in}}-h_{\mathrm{exp,out,is}}} \]

Generated power:

\[ W_{\mathrm{exp}} = \dot{m}_{\mathrm{exp}} \left( h_{\mathrm{exp,in}}-h_{\mathrm{exp,out}} \right) \]

Heat exchanger

Energy balance kati ya hot na cold fluids:

\[ \dot{m}_{h} \left( h_{h,\mathrm{in}}-h_{h,\mathrm{out}} \right) = \dot{m}_{c} \left( h_{c,\mathrm{out}}-h_{c,\mathrm{in}} \right) \]

Heat-exchanger pressure-loss coefficient:

\[ f_p = \frac{\Delta p}{p_{\mathrm{in}}} \]

Kwa sababu temperature change katika real fluids si linear, heat exchangers ziligawanywa katika equal-enthalpy elements. Katika kila node fluid temperatures zilihesabiwa kutoka REFPROP database na minimum approach temperature ikakaguliwa throughout whole heat exchanger. Method hii inalenga kuzuia physically impossible temperature crossovers.

Storage tank

Storage tank volume:

\[ V_{\mathrm{tank}} = \frac{\dot{m}_{s}t}{\rho} \]

• V_tank: tank volume, m³,
• ṁ_s: mass flow rate ya storage medium, kg/s,
• t: charge or discharge duration, s,
• ρ: density ya storage medium, kg/m³.

Economic model inategemea assumptions zipi?

Equipment costs za compressor, expander, heat exchangers, storage tanks, motor na generator zilihesabiwa kwa literature-based cost functions. Costs kutoka different years ziliupdatewa kwa Chemical Engineering Plant Cost Index iliyokubaliwa kuwa 699,97 kwa 2021:

\[ Z_k = Z_{\mathrm{original}} \left( \frac{\mathrm{CEPCI}_{2021}} {\mathrm{CEPCI}_{\mathrm{original}}} \right) \]

Total purchased-equipment cost:

\[ Z_{\mathrm{PC}} = \sum_{k=1}^{n} Z_k \]

Economic assessment assumptions zilikuwa:

• Plant life: 30 years,
• Discount rate: %10,
• Annual operation and maintenance cost: %3 of total capital cost,
• Electricity cost: 0,025 USD/kWh,
• 365 full charge-discharge cycles per year in static model.

Round-trip efficiency na LCOS zimefafanuliwaje?

Design-point round-trip efficiency ni ratio ya net electricity obtained during discharge kwa net electricity consumed during charge:

\[ \chi = \frac{ \dot{m}_{\mathrm{dis}}t_{\mathrm{dis}} \left( W_{\mathrm{dis,exp}}-W_{\mathrm{dis,com}} \right) }{ \dot{m}_{\mathrm{chr}}t_{\mathrm{chr}} \left( W_{\mathrm{chr,com}}-W_{\mathrm{chr,exp}} \right) } \]

Levelized cost of storage:

\[ \mathrm{LCOS} = \frac{ \mathrm{FCR}\cdot Z+ \mathrm{O\&M}+ E_{\mathrm{in}}P_{\mathrm{elc}} }{ E_{\mathrm{out}} } \]

• FCR: fixed annual capital-recovery rate,
• Z: total investment cost,
• O&M: annual operation and maintenance cost,
• E_in: annual charge electricity, kWh,
• P_elc: unit electricity cost,
• E_out: annual discharge electricity, kWh.

LCOS inaonyesha ni kiasi gani cha investment, maintenance na energy cost kinahusishwa na kila kilowatt-hour ya discharge electricity inayozalishwa na system. Annual discharge amount ikipungua, fixed investment cost hugawanywa kwa energy kidogo zaidi, hivyo LCOS hupanda.

Thermodynamic na economic models zilivalidatewaje?

Thermodynamic model ilivalidatewa kwa recalculating na comparing previously published low- na high-temperature supercritical CO₂ PTES systems. Kwa low-temperature system, source round-trip efficiency ilikuwa %60,04 na model ikatoa %59,86; kwa high-temperature system source value ilikuwa %78,40 na model value %78,24. Relative differences zilikuwa respectively approximately %0,3 na %0,2.

Economic model ilicomparewa na another PTES study. Source LCOS ilikuwa 0,243 USD/kWh, na katika model hii ikahesabiwa 0,248 USD/kWh, relative difference %2,06.

Comparisons hizi zinaonyesha model inaweza reproduce reference studies approximately. Hata hivyo, validation hii haimaanishi 8.760-hour operation ya study hii imecomparewa na experimental au commercial PTES plant.

Wind-solar microgrid ilimodeliwaje?

Case scenario inawakilisha typical wind na solar resources katika Qinghai, China. Total installed renewable power ni 600 MW:

• Wind: 300 MW,
• Photovoltaic: 300 MW.

Hourly wind na solar generation zilitolewa kutoka MERRA-2 meteorological reanalysis data na kuadjustiwa kwa UTC+8 time zone. Local consumption iliwakilishwa na synthetic daily profile yenye industrial base load dominant. Peak load ni 70 MW na %3 random disturbance iliongezwa kuonyesha small operational fluctuations.

Maximum external-grid transfer power ni 80 MW. Limit hii si mandatory export schedule. Kwa hiyo PTES haidischarge ili kufidia saa ambazo 80 MW haiwezi kutumwa external grid; mainly discharge inalenga kukidhi local load deficit.

Renewable-energy surplus na local deficit zinahesabiwaje?

Total power ambayo microgrid inaweza kukubali katika one hour:

\[ P_{\mathrm{acc}}(t) = P_{\mathrm{demand}}(t)+P_{\mathrm{export}} \]

Total renewable generation:

\[ P_{\mathrm{RE}}(t) = P_{\mathrm{wind}}(t)+P_{\mathrm{PV}}(t) \]

Renewable-energy surplus inayopatikana kwa PTES charging:

\[ P_{\mathrm{sur}}(t) = \max \left[ 0,\, P_{\mathrm{RE}}(t)-P_{\mathrm{acc}}(t) \right] \]

Local electricity deficit:

\[ P_{\mathrm{def}}(t) = \max \left[ 0,\, P_{\mathrm{demand}}(t)-P_{\mathrm{RE}}(t) \right] \]

PTES inaweza charge wakati renewable generation inazidi sum ya local load na 80 MW external-transfer capacity. Renewable generation ikishuka directly below local load, PTES inaweza discharge.

State of charge ya storage tank ilifuatiliwaje?

Model inafuatilia energy state ya system kupitia usable heat katika high-temperature hot storage tank:

\[ \mathrm{SOC}(t) = \frac{ E_{\mathrm{HHST}}(t) }{ E_{\mathrm{HHST,max}} } \]

Baada ya one-hour step, usable heat katika tank:

\[ E_{\mathrm{HHST}}(t+\Delta t) = E_{\mathrm{HHST}}(t) \left( 1-\delta\Delta t \right) + P_{\mathrm{ch}}(t) \mathrm{COP} \left( \mathrm{PLR}_{\mathrm{ch}} \right) \Delta t - \frac{ P_{\mathrm{dis}}(t) }{ \eta_{\mathrm{he}} \left( \mathrm{PLR}_{\mathrm{dis}} \right) } \Delta t \]

• δ: hourly equivalent heat-loss coefficient, 5 × 10⁻⁴ hour⁻¹,
• P_ch: actual charge electric power,
• P_dis: actual discharge electric power,
• COP: part-load-dependent charge-cycle coefficient of performance,
• η_he: part-load-dependent heat-engine efficiency.

SOC value imelimitwa kati ya 0 na 1. Tank ikiwa full, new charging haiwezekani; ikiwa empty, new discharging haiwezekani. Initial SOC value ni 0,10.

Kwa nini minimum part-load limit inahitajika?

Imeassumiwa kuwa compressor na expander haziwezi kufanya kazi stably na efficiently katika very low powers. Kwa hiyo minimum effective part-load ratio:

\[ \mathrm{PLR}_{\min}=0.20 \]

imewekwa. Available charging surplus au local load deficit ikiwa below %20 ya nominal power, system inaweza kubaki off badala ya kufanya kazi kwa low power.

Part-load performance loss imemodeliwaje?

Isentropic efficiency ya compressor na expander at part load imeadjustiwa kwa empirical quadratic penalty function:

\[ \eta_{\mathrm{com/exp}}(\mathrm{PLR}) = \eta_{\mathrm{com/exp,des}} \left[ 1-\zeta \left( 1-\mathrm{PLR} \right)^2 \right] \]

• η_des: design-point isentropic efficiency,
• PLR: ratio of actual power to nominal power,
• ζ: part-load degradation coefficient.

Main calculations zilitumia ζ = 0,3. ζ = 0,1 inawakilisha weak na ζ = 0,5 strong performance degradation sensitivity scenarios. Equation hii si manufacturer-specific compressor au turbine map.

Katika kila PLR value si individual equipment efficiency tu ilibadilishwa; whole charge na discharge cycle zilirecalculatewa. Baadaye system-level COP na heat-engine efficiency zilifit kwenye curves zifuatazo:

\[ \mathrm{COP}(\mathrm{PLR}) = -0.3421\mathrm{PLR}^2 + 0.7073\mathrm{PLR} + 1.0962 \]

\[ \eta_{\mathrm{he}}(\mathrm{PLR}) = -0.1883\mathrm{PLR}^2 + 0.3767\mathrm{PLR} + 0.2682 \]

Kwa PLR = 1, COP ni approximately 1,461. Heat-engine fit curve inatoa approximately 0,4566 huku design value ikiwa 0,4584. PLR inapokaribia %20, COP hushuka hadi approximately 1,22 na heat-engine efficiency hadi approximately 0,336.

“Quasi-dynamic” term ina maana gani?

Model inafuatilia kila saa ya mwaka katika chronological order. Kwa each hour, wind generation, solar generation, local load, electricity surplus au deficit, SOC, charge-discharge power, PLR na cycle efficiency zinaupdatewa.

Hata hivyo, ndani ya one-hour time step system inaassumiwa kuwa steady-state. Model haisolve kwa detail processes zifuatazo:

• Second-scale acceleration ya compressor na expander,
• Start-up and shutdown energy consumption,
• Transient pressure and temperature waves in pipes,
• Development ya temperature stratification katika storage tanks ndani ya saa,
• Transient regimes za control valves na motor-generator.

Kwa hiyo method si full dynamic physical simulation; ni hourly quasi-dynamic dispatch model inayofuatilia chronological constraints.

Dynamic round-trip efficiency ilihesabiwaje?

Ratio ya annual actual discharge electricity kwa annual actual charge electricity:

\[ \mathrm{RTE}_{\mathrm{dyn}} = \frac{ \sum_{t=1}^{8760} P_{\mathrm{dis}}(t)\Delta t }{ \sum_{t=1}^{8760} P_{\mathrm{ch}}(t)\Delta t } \times 100\% \]

Indicator hii inapima electricity-to-electricity conversion ya actual annual operating sequence badala ya efficiency ya single cycle at design point.

Recovery ratio ya curtailed renewable energy ni nini?

Ratio ya renewable-energy surplus actually absorbed by PTES kwa total available surplus:

\[ R_{\mathrm{abs}} = \frac{ \sum_{t=1}^{8760} P_{\mathrm{ch}}(t)\Delta t }{ \sum_{t=1}^{8760} P_{\mathrm{sur}}(t)\Delta t } \times 100\% \]

Ratio hii inapoongezeka, system inaweza kuhifadhi larger fraction ya energy iliyo katika curtailment risk. Hata hivyo, higher recovery haimaanishi kila mara lower cost; kunasa rare very-high-power peaks kunaweza kuhitaji equipment kubwa inayobaki unused au at low load kwa sehemu kubwa ya mwaka.

Reference six-hour system ilifanyaje kazi mwaka mzima?

Figure 2(a) inaonyesha SOC ya high-temperature hot tank ikisogea mara kwa mara kati ya 0 na %100 katika mwaka mzima. System ilifikia repeatedly fully full au fully empty limits.

Tank kuwa almost full kwa roughly one-third of year kunalimit capacity ya kukubali new renewable-energy surplus. Kuwa almost empty kwa approximately one-fifth of year kunalimit ability ya kusaidia local electricity deficit. Six-hour storage inaweza kutoa hourly na intraday shifting, lakini haiwezi kuadapt fully kwa longer-duration source-load mismatches.

Typical high-variability week inaonyesha nini?

Figure 3 inaonyesha high-variability week kati ya hours 646 na 813 za year. Peaks za wind na solar generation katika some hours zinazidi 400 MW, huku acceptance curve iliyoundwa na local load na external-transfer limit ikibadilika approximately kati ya 50-150 MW.

Renewable generation inapozidi acceptance limit, charging power huongezeka lakini inalimitiwa na 149,34 MW nominal limit. Local generation ikishuka below load, discharge hutokea. Charge na discharge powers hazikai fixed at nominal values kwa muda mrefu; many hours zinaoperate at medium au low power.

Visual hii inaonyesha directly kwa nini annual performance haiwezi kuhesabiwa kwa constant %66,92 efficiency. Kwa sababu renewable surplus, load deficit na SOC zinabadilika kila saa, compressor na expander pia zinaoperate continuously katika different PLR values.

Compressor na expander zilifanya kazi katika loads zipi?

Figure 4(a) inaonyesha compressor PLR values zimespread between %20 na %100. Compressor ina two distinct operating groups:

• Near-full-load periods ambapo high renewable-energy surplus inafyonzwa,
• Medium-low-load tracking periods around %45-%60.

Compressor ilifanya kazi near full load kwa only approximately %20 ya effective operating time.

Expander load distribution ilishift kwenda lower values. Highest expander PLR ni approximately %67 na for more than half ya effective operating time PLR iko around below %45. Sababu ni kwamba discharge power inalimitiwa na both local load deficit na usable heat available katika tank.

Cycle performance ilibadilikaje katika mwaka?

Figure 5(a) inaonyesha COP na heat-engine efficiency values katika annual effective operating hours:

• Charge COP mostly in 1,22-1,46 range.
• Discharge heat-engine efficiency mostly in 0,34-0,43 range.
• Most operating points ziko below design values approximately 1,461 na 0,4584.

Figure 5(b) inaonyesha both indicators zinashuka kadri PLR inavyopungua. Hii inaonyesha source-load variability haibadilishi only wakati system inafanya kazi, bali pia quality ya electricity-heat-electricity conversion wakati wa operating hours.

Difference kati ya static na quasi-dynamic assessment ni kubwa kiasi gani?

Cost components katika Figure 6 chini ya static assessment ni approximately:

• Investment cost: 0,0763 USD/kWh,
• Electricity cost: 0,0374 USD/kWh,
• Operation and maintenance: 0,0229 USD/kWh.

Katika quasi-dynamic assessment:

• Investment cost: 0,1773 USD/kWh,
• Electricity cost: 0,0448 USD/kWh,
• Operation and maintenance: 0,0532 USD/kWh.

Investment na maintenance amounts si lazima physically ziongezeke more than twofold. Main reason per-unit-energy values zinapanda ni fixed costs kugawanywa kwa only 94,20 GWh discharge electricity badala ya 219 GWh katika static model.

Power na energy capacity zilioptimized vipi?

Two decision variables zilitumika:

• Power scaling coefficient λ: 0,2-1,5 in steps of 0,1,
• Storage duration t_st: 2-12 hours in 1-hour steps.

Charge na discharge powers ziliscalewa together:

\[ P_{\mathrm{ch/dis,r}}(\lambda) = \lambda P_{\mathrm{ch/dis,base}} \]

High-temperature storage capacity ilihesabiwa kutoka nominal discharge power na storage duration:

\[ E_{\mathrm{HHST,max}} = \frac{ P_{\mathrm{dis,r}}(\lambda)t_{\mathrm{st}} }{ \eta_{\mathrm{he,des}} } \]

Power- na energy-related investment costs ziliscalewa kwa capacity kwa kutumia 0,8 exponent relation:

\[ Z_{\mathrm{power}}(\lambda) = Z_{\mathrm{power,base}} \lambda^{\alpha} \]

\[ Z_{\mathrm{energy}}(\lambda,t_{\mathrm{st}}) = Z_{\mathrm{energy,base}} \left( \lambda \frac{t_{\mathrm{st}}}{t_{\mathrm{base}}} \right)^{\alpha} \]

\[ \alpha=0.8 \]

Lowest LCOS ilipatikana katika configuration gani?

Three-dimensional LCOS surface katika Figure 7 inaonyesha lowest-cost region iko katika low-power long-duration region, si high-power short-duration structure.

Power ikipunguzwa, system haiwezi kunasa all highest renewable-energy surplus peaks. Kwa hiyo recovery ratio hushuka kutoka %36,84 katika reference configuration hadi %21,53. Kwa upande mwingine, equipment hufanya kazi kwa hours nyingi zaidi closer to nominal power, part-load penalty hupungua na dynamic efficiency huongezeka kutoka %55,81 hadi %64,37.

Twelve-hour storage duration huruhusu low-power configuration kusawazisha energy mismatches katika longer intervals. Hivyo utilization ya smaller power equipment huongezeka huku sufficient thermal-energy capacity ikihifadhiwa.

Kwa nini capturing maximum renewable-energy surplus si most economical objective?

Pareto frontier katika Figure 9 inaonyesha nonlinear conflict kati ya LCOS na renewable-energy-surplus recovery ratio. Baada ya minimum-cost point, capacity ikiongezwa moderately, recovery ratio inaweza kupanda kwa limited cost increase.

Hata hivyo, recovery ratio ikisukumwa kwenda higher levels, high nominal power inahitajika kunasa large power peaks zinazoweza kutokea only a few times per year. High power hii hubaki unused au inaoperate at low PLR kwa sehemu kubwa ya year. Matokeo yake:

• Capacity utilization hupungua,
• Part-load losses huongezeka,
• Fixed-investment share per unit energy hupanda,
• LCOS hupanda rapidly.

Kwa hiyo study inaargue kwamba sizing storage solely to capture all curtailed energy haipaswi kuwa only objective.

Results ziliathirikaje part-load penalty coefficient ilipobadilika?

Strong part-load loss iliharibu performance ya reference system kwa kiasi kikubwa. Pamoja na hayo, optimum power scale na storage duration hazikubadilika katika all three ζ values. Hii inaonyesha tendency kwenda low-power long-duration storage haitegemei solely chosen ζ = 0,3.

Hata hivyo, sensitivity analysis hii haichukui nafasi ya manufacturer maps. Inaonyesha only jinsi result ilivyo stable ndani ya selected empirical function.

Wind na solar composition ilibadilishaje optimum system?

Researchers walilinganisha three generation structures chini ya same total 600 MW installed capacity:

• 300 MW wind + 300 MW solar,
• 600 MW equivalent photovoltaic,
• 600 MW equivalent wind.

Pure-solar na pure-wind profiles zilipatikana kwa scaling corresponding generation series katika hybrid scenario kulingana na installed capacity. Si independent measurement series kutoka different sites.

Pure photovoltaic scenario ilitoa lowest LCOS. Figure 11 inaonyesha charging PLR distribution katika scenario hii imeconcentrate katika %90-%100 region. Solar-energy surplus huunda more concentrated high-power packages during daytime hours with high irradiance, hivyo PTES inaweza kufanya kazi near nominal charging power more frequently.

Wind na hybrid scenarios zina longer low-load tails katika power surplus. Especially katika pure-wind scenario available surplus energy ni more irregular na low-density, hivyo effective operating time na capacity utilization hupungua na LCOS hupanda.

Dynamic RTE differences kati ya scenarios ni relatively small. Major economic difference haisababishwi mainly na basic conversion efficiency ya cycle yenyewe, bali na jinsi renewable-energy surplus ilivyo concentrated over time na jinsi ilivyo close to nominal power.

Scientific message ya figures ni nini?

• Figure 1: Inaonyesha transfer ya wind na solar electricity kupitia CO₂ Brayton cycle kwenda hot na cold storage, kisha regeneration ya electricity.
• Figure 2: Inaonyesha six-hour reference tank ikifikia fully full na fully empty limits mara nyingi katika year.
• Figure 3: Inaonyesha charge na discharge power zikibadilika continuously na renewable generation, load na SOC wakati wa high-variability week.
• Figure 4: Inaonyesha compressor na expander zikifanya kazi far from design point kwa large share ya annual operating time.
• Figure 5: Inaonyesha COP na heat-engine efficiency zinashuka PLR inapopungua na annual performance kuunda scattered distribution.
• Figure 6: Inaeleza static model inapunguza LCOS kwa kuspread investment na maintenance costs juu ya higher annual discharge energy.
• Figure 7: Inaonyesha minimum LCOS iko katika low-power na long-storage-duration region.
• Figure 8: Inaonyesha reducing power huruhusu equipment kuoperate at higher PLR na kuongeza dynamic RTE.
• Figure 9: Inaonyesha Pareto conflict kati ya low cost na high renewable-energy recovery.
• Figure 10: Inalinganisha optimum power, duration, LCOS na recovery ratio kwa hybrid, pure-solar na pure-wind generation.
• Figure 11: Inaonyesha charging power imeconcentrate zaidi katika high-PLR region katika photovoltaic scenario.

Results zinazoungwa mkono na study ni zipi?

• Katika model hii, design-point efficiency inaoverstate realistic annual operating efficiency kwa kiasi kikubwa.
• SOC constraints na source-load temporal mismatch huzuia full utilization ya nominal storage capacity throughout year.
• Long operation ya compressor na expander at low PLR hupunguza cycle efficiency.
• Annual discharge ya reference six-hour, 100 MW system ilibaki approximately %43 ya static estimate.
• Reducing nominal power na extending storage duration kunaweza kuongeza equipment utilization na dynamic RTE huku LCOS ikipungua.
• Hourly power distribution ya surplus, pamoja na annual total renewable energy, huamua optimum storage capacity.
• Katika examined scenarios, pure photovoltaic generation ilitoa lowest LCOS kutokana na more concentrated high-power charging periods.

Study haithibitishi nini?

• Haionyeshi kwamba 100 MW molten-salt CO₂-PTES plant ilijengwa na kuoperatewa actually katika field.
• Haionyeshi kwamba 0,1772 USD/kWh ni commercial bid au guaranteed real-plant cost.
• Haivalidate compressor na expander performance kwa manufacturer test maps.
• Haimodel start-up/shutdown losses, transient thermal stresses au control-system delays.
• Haivalidate wind, solar na load profiles kwa simultaneous meter data measured katika real microgrid.
• Haichunguzi hourly electricity-market price variation au revenue optimization.
• Haimodel price katika external-transfer channel, mandatory export schedule au grid-service revenues.
• Haionyeshi kwamba recovering all renewable-energy surplus ni technically au economically best objective.
• Haithibitishi pure-solar scenario itakuwa more economical than pure wind katika all geographies.

Nguvu za utafiti ni zipi?

Main strength ni kuunganisha thermodynamic cycle model na chronological microgrid operation katika same framework. Model haiangalii annual energy totals pekee; inahifadhi sequence ya 8.760 hours na kuhamisha tank SOC kutoka saa moja kwenda nyingine.

Kuhamisha compressor na expander part-load losses kwanza kwenda equipment efficiency na kisha kwenda whole-cycle COP na heat-engine efficiency ni more detailed kuliko treating storage as constant-efficiency box.

Kuhesabu static na quasi-dynamic LCOS separately kwa same system kunafanya economic bias kutoka design-point approach ionekane. Two-dimensional scan ya power na storage duration pamoja na extraction ya Pareto frontier pia zinaonyesha cost-benefit conflict badala ya kupendekeza one arbitrary capacity.

Main limitations ni zipi?

• Study ni preprint ambayo haijapitia peer review.
• Wind na solar generation zilimodeliwa kutoka meteorological reanalysis; local load iligeneratewa synthetically.
• Part-load penalty function ni empirical na haitegemei actual manufacturer map.
• Model time resolution ni one hour; short-term dynamics hazifuatiliwi.
• Start-up, shutdown na mode-switching losses hazijahesabiwa explicitly.
• Only high-temperature hot tank inafuatiliwa kama SOC indicator; detailed filling na temperature distributions za other tanks hazijaripotiwa.
• Equivalent constant hourly heat loss imetumika kwa storage tank.
• Pipe na tank pressure losses zimepuuzwa katika basic thermodynamic model.
• Tank fluids zimeassumiwa perfectly mixed.
• Electricity price imeassumiwa 0,025 USD/kWh kwa all periods; hourly market prices hazijachunguzwa.
• Single 0,8 scaling exponent imetumika kwa both power na energy equipment katika investment costs.
• Hakuna probabilistic analysis kwa cost uncertainty, exchange-rate change, financing structure au equipment learning curve.
• Pure-solar na pure-wind scenarios si independent meteorological datasets bali equivalent scaled forms za existing profiles.
• REFPROP version imeandikwa 9.0 na 9.1 katika two different places kwenye text.
• Hakuna external validation kwa annual operation data ya experimental au commercial PTES plant.

Study ina maana gani kwa past, present na future?

Sehemu kubwa ya past PTES research imefocus kwenye pressure ratio, fluid, temperature na heat-exchanger design zinazomaximize thermodynamic efficiency. Approach hii ni muhimu kuelewa physical potential ya cycle, lakini peke yake haionyeshi system ya real renewable-linked itatumika kiasi gani katika year.

Study hii inatathmini storage value si kupitia “best cycle efficiency” pekee bali kupitia hourly source-load matching, SOC, equipment load na annual cost. Most important message ni kwamba larger power au higher renewable-energy capture ratio haimaanishi automatically better economic performance.

Future models zinaweza kuongeza real compressor na expander performance maps, experimental tank heat losses, shorter time steps, start-up/shutdown processes na real-plant load data. Hourly electricity prices, capacity payments na grid services zikijumuishwa, value ya PTES inaweza kuchunguzwa si kama curtailment-reduction device pekee bali kama asset inayotoa different market services.

Mbinu na Matokeo ya Utafiti

Technical method summary

Annual operating indicators za reference system

Static na dynamic economic comparison

Main optimization result

Optimum system licha ya kufyonza less renewable-energy surplus imefikia higher capacity utilization, higher PLR na lower unit energy cost. Main optimization result ni kwamba system “capturing the most energy” na system yenye “lowest cost” si lazima ziwe same.

Maelezo ya Chanzo na Mbinu

Full original title: Quasi-dynamic techno-economic optimization of Brayton-cycle pumped thermal electricity storage for wind-solar microgrids considering part-load performance degradation

Authors and order: Di Qi; Yuting Wu; Xu Feng; Yanjun Du; Cancan Zhang

Co-first author or equal contribution: PDF haina co-first-authorship au equal-contribution statement.

Contact author: Xu Feng. Name imeonyeshwa kwa asterisk katika PDF na listed as “Contact Author” katika official SSRN record.

Contact-author email: Hakuna visible email address katika uploaded PDF.

Institutional affiliations:

1. Beijing Key Laboratory of Heat Transfer and Energy Conversion, National User-Side Energy Storage Innovation Research and Development Center, Beijing University of Technology, Beijing 100124, China
2. Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China

DOI: 10.2139/ssrn.6946540

Publication platform: SSRN

Official link:https://ssrn.com/abstract=6946540

Journal or conference: No peer-reviewed journal or conference publication has been verified for this version.

Original peer-reviewed publisher: No peer-reviewed journal publisher information is available. Study is distributed as preprint via SSRN.

Publication year: 2026

Exact SSRN upload date: Exact upload date could not be verified from accessible official record information.

Source type: Preprint research article including thermodynamic model, hourly chronological operation simulation, techno-economic assessment and capacity optimization

Peer-review status: Study haijapitia peer review.

Author contributions: Uploaded PDF haina detailed CRediT au author-contribution statement.

Funding: Frontier Technologies R&D Program of Jiangsu, project number BF2025054.

Conflict of interest: No separate conflict-of-interest statement was identified in uploaded version.

Data access: Study states hourly wind na photovoltaic profiles were generated using MERRA-2-based methods. Full time series na simulation code hazijasharewa katika uploaded PDF.

Makala hii ya Kituruki ya Verianla iliandaliwa kwa kuchunguza entire text, mathematical equations, tables, charge-discharge schematics, SOC graphs, PLR distributions, LCOS comparison, capacity surfaces, Pareto analysis na generation scenarios katika uploaded PDF. Hakuna new thermodynamic performance, cost, real-plant success au renewable-energy recovery claim kutoka outside PDF iliyoongezwa.

Main limitations ni lack of peer review, inability ya hourly quasi-dynamic model to capture short-term transients, wind-solar generation being derived from reanalysis data na load from synthetic profile, modeling ya part-load losses kwa empirical function rather than manufacturer map, lack of external validation with real PTES plant data, na absence ya probabilistic economic-uncertainty analysis.

PDF inasema REFPROP 9.0 kwa heat-exchanger property calculations na REFPROP 9.1 katika general model description. Pia discharge-efficiency fit equation inatoa approximately %45,66 at PLR = 1, huku design table ikitoa %45,84. Small version na fit differences hizi zinapaswa kuzingatiwa katika interpretation.

Optimum LCOS value 0,1772 USD/kWh ilihesabiwa chini ya stated meteorological profile, synthetic load, 80 MW transfer limit, 0,025 USD/kWh electricity cost, %10 discount rate, %3 annual maintenance cost, 30-year lifetime na other model assumptions. Haipaswi kutafsiriwa kama guaranteed cost kwa real commercial plant.

Study hii ni preprint ambayo haijapitia peer review; results zinapaswa kusomwa kwa kuzingatia limitation hii.