What Matters Most in Large-Scale Solar Installations | Site Selection, Grid Integration, System Design
Site selection needs annual irradiation ≥ 1500 kWh/m², reduce 5% shading loss; grid connection should configure ≥ 99% efficiency inverter and energy storage to smooth fluctuation; system design optimize DC/AC ratio 1.2–1.4 to elevate overall power generation revenue.
Site Selection
Measure land
Developing 100 megawatts level's large-scale ground photovoltaic project needs to occupy 250 hectares to 300 hectares' physical space. Early stage survey needs to adopt carrying LiDAR's drone to conduct mapping, vertical precision error needs to be controlled at 5 centimeters within, horizontal precision error controlled at 10 centimeters within, point cloud density reaches per square meter 50 points.
When the site's original terrain slope exceeds 10% (about 5.7 degrees), per hectare's earthwork leveling amount will increase 1500 cubic meters to 2000 cubic meters, leading to civil construction budget floating up 25%. Fixed bracket system capable of tolerating maximum slope is 15% (about 8.5 degrees), flat single-axis tracking bracket's fault tolerance rate is low, requires south-north direction slope smaller than 5% (about 2.8 degrees), east-west direction slope smaller than 10%.
Exceeding limit value will let tracking main shaft torque overload, motor lifespan from expected's 15 years shortened to less than 5 years. Site inside if exists exceeding 2 meters deep gully, backfill compaction degree needs to reach 90% above, otherwise later stage bracket foundation settlement amount exceeding 30 millimeters will trigger module frame stress deformation.
Probe soil structure
Underground 1.5 meters to 3.0 meters's geological structure controlled foundation piling's engineering progress and construction plan.
· Soil bearing capacity test needs on site inside according to every 20 hectares 1 hole's density to conduct drilling, standard penetration test (SPT) blow count N value greater than 15, soil foundation bearing capacity exceeding 100 kilopascals (kPa).
· If at underground 1.0 meter place encountering rock layer, static pressure piling machine unable to operate, needs to adopt down-the-hole drill to pre-drill hole, per megawatt's construction cost will extra increase 20,000 US dollars to 30,000 US dollars, single piece pile's construction operation time from average 3 minutes extended to 15 minutes.
· Pull-out force test segment, single piece micropile's anti-pull bearing capacity limit value needs to reach 20 kilonewtons (kN), horizontal bearing capacity needs to reach 15 kilonewtons (kN).
· Soil corrosivity sampling laboratory test inside, if pH value is lower than 5.5 or sulfate concentration is greater than 500 milligrams/kilogram (mg/kg), steel material's annual corrosion rate exceeding 0.05 millimeters, galvanized layer's thickness needs to increase from standard's 65 micrometers to 85 micrometers, or change to use zinc-aluminum-magnesium alloy material, causing metal structure procurement budget increasing 12%.
· Consulting past 20 years's meteorological statistics, if the maximum frozen soil depth exceeds 60 centimeters, the pile foundation's buried depth needs to correspondingly increase 40 centimeters, and anti-frost heave design will let steel overall usage amount elevate 5%.
Calculate illumination
Relying on the past 15 years' satellite irradiance data set's error rate is between 8% to 12%. Needs on site to install at least 3 sets of solar measurement stations, collect continuous 12 months, time interval being 10 minutes' hourly ground actual measured samples, including global horizontal irradiance (GHI), direct normal irradiance (DNI) and diffuse horizontal irradiance (DHI). Solar measurement station configured's first-class pyranometer needs every 14 days to conduct one time manual cleaning, maintain measurement precision error at 2% below.
Whole year GHI reading greater than 1400 kilowatt-hours/square meter (kWh/m²), only then can let project's capacity factor maintain at 22% above. Measuring ground surface albedo to bifacial module's power generation prediction has significant parameter impact. Bare soil's albedo reading normally is between 0.15 to 0.20, dry grassland is 0.25. If the site has paved white crushed stone, albedo can elevate to 0.50 to 0.60. Albedo every increases 0.10, bifacial module's back side gain elevates 2.5%, pulling total levelized cost of energy (LCOE) descending 1.8%.
Flip through hydrological records
The Water Affairs Department issued a 100-year return period flood inundation model, and the inundated area's water depth highest value needs to be smaller than 0.3 meters. When the historical flood water level exceeds 0.5 meters, the photovoltaic module's from the ground's lowest edge needs to be elevated to 0.8 meters above. Pillar length increases by 0.3 meters, structure bearing's limit wind load overturning moment increases 15%, supporting profile's cross-sectional thickness needs to increase 10%, per megawatt's steel weight increases about 4.5 tons.
The site drainage system needs to design a bottom width of 500 millimeters, a depth of 500 millimeters's trapezoidal concrete intercepting ditch, 50 megawatts scale needs to excavate 3000 meters to 5000 meters's ditch, corresponding engineering expenditure is in the 150,000 US dollars to 200,000 US dollars interval. The underground water level elevation needs to be located at ground surface below 1.5 meters. If the underground water level is at 0.5 meters depth, a crawler type piling machine's ground specific pressure exceeding 35 kilopascals (kPa) will cause subsidence, needs to pave a 200 millimeters's crushed stone cushion layer, pushing the high about 8% temporary facility budget.
Calculate traffic fee
Fifty megawatts's equipment shipping amount involves 250 to 300 standard 40-foot containers. Entering site main road's minimum turning radius reaches 15 meters, roadbed's maximum bearing capacity reaches 30 tons. From highway exit to construction site's unpaved dirt road distance if exceeding 5 kilometers, heavy-duty truck's single trip driving time increases 45 minutes, logistics company will add terminal transportation charge per container 200 US dollars's loading and unloading fee.
Site inside maintenance road's width minimum keeps 4.5 meters, satisfying 65-ton class truck crane's driving passage. Road's longitudinal slope lower than 8%, rainy day muddy state under friction coefficient smaller than 0.3 time, flatbed trailer has 25% probability of happening side slip derailing. Paving a 150-millimeter thick graded crushed stone layer can increase road surface adhesion, unit price cost is about 15 US dollars per square meter.
Grid Integration
Probe transformer
Developing a 100 megawatts level project before, needs to retrieve the target substation's past 36 months's 8,760 hours running log. If the point of interconnection (POI)'s 132 kilovolts (kV) substation main transformer rated capacity is 150 megavolt-amperes (MVA), the current peak load has already reached 110 megavolt-amperes, and the remaining margin is only 40 megavolt-amperes. Accessing 100 megawatts capacity will lead to transformer thermal overload protection tripping. At this time, the need to toward local power grid operator apply main transformer expansion engineering, procure one unit brand new's 100 megavolt-amperes double winding transformer, equipment's manufacturing delivery cycle as long as 18 months to 24 months, including civil construction and equipment installation's total budget as high as 2.5 million US dollars to 3.5 million US dollars.
Power grid strength test indicators inside, short circuit ratio (SCR) lower than 3.0's node is defined as weak power grid. When the short circuit ratio is located in the 1.5 to 2.0 interval, the photovoltaic inverter's phase-locked loop (PLL) circuit will suffer a 0.2 seconds above's voltage drop time, losing synchronization, triggering large-scale grid disconnection. Needs at booster station alternating current side add install one unit capacity being 30 megavolt-amperes reactive (MVAr) static synchronous compensator (STATCOM), equipment procurement unit price is about per megavolt-ampere reactive 45,000 US dollars, extra plus every year 20,000 US dollars's operation and maintenance fee.
Pull wires
Transmission line's physical distance and voltage level planning to early stage capital expenditure (CAPEX) produces huge impact.
· When the booster station to the point of interconnection's straight line distance is 8 kilometers, adopting 33 kilovolts (kV) medium voltage overhead line sending out, conductor selects cross-sectional area being 400 square millimeters's aluminum conductor steel reinforced (ACSR), per kilometer's comprehensive construction cost is about 80,000 US dollars to 120,000 US dollars.
· Distance breaking through 15 kilometers boundary, 33 kilovolts line's line loss rate will from 1.2% soar to 3.5% above, leading to every year losing about 4.5 million degrees of electricity's revenue. Must put sending out voltage upgrade to 132 kilovolts or 220 kilovolts high voltage level, single circuit line's construction cost following it increases to per kilometer 200,000 US dollars to 300,000 US dollars.
· Overhead line along the line's right of way (ROW) width needs to keep in the 30 meters to 40 meters range, if passing through private forest land or nature reserve, environmental protection assessment and land right-of-way negotiation cycle needs to consume time 12 months to 18 months, compensation fee approximately occupies line total construction cost's 15%.
· Line crossing highway or navigable river time, iron tower height needs to be from conventional's 24 meters to 45 meters above, wind load calculation needs to resist per second 45 meters's maximum gust, single base tension tower's steel material usage amount from 8 tons soars to 22 tons, foundation pit excavation depth increases to 4.5 meters.
Stabilize voltage
Inverter array at full load 100 megawatts output time, power factor (PF) adjustment range needs to cover leading 0.95 to lagging 0.95. When the power grid voltage suddenly drops to 20% of rated value, the inverter must startup low voltage ride through (LVRT) function, in 150 milliseconds inside toward power grid inject rated current 1.05 times's reactive current, and maintaining uninterrupted grid-connected running at least 3 seconds under abnormal state. The total harmonic distortion (THD) parameter needs to be strictly monitored, and the point of interconnection's current harmonic distortion rate highest limit value set as 3%, even harmonic current module cannot exceed fundamental wave current's 1%.
If 100 units of 1 megawatt rated power inverters work at the same time, produce harmonic amplification effect, triggered high frequency resonance will lead to 33 kilovolts switchgear inside's lightning arrester in 48 hours because of heat accumulation breakdown damage. Voltage flicker limit aspect, short-term flicker severity (Pst) reading needs to be smaller than 1.0, long-term flicker severity (Plt) needs to be limited at 0.8 within, ensuring surrounding 5 kilometers radius inside's industrial user motor equipment will not because of voltage fluctuation appear rotation speed descending or shutting down.
Check consumption rate
Through power flow calculation software, establish regional power grid topology model, simulate future 10 years different seasons load valley period's power transmission situation. Because daytime noon time period photovoltaic output reaches peak value, while residential and commercial electricity usage load locates at valley, unavoidably will produce curtailment phenomenon. If the financial model inside the set's feed-in tariff is 0.05 US dollars/degree, at different curtailment rate conditions, under's revenue loss data manifests extremely big difference.
Simulation year | Regional expected total installed capacity (megawatts) | Transmission line maximum transmission threshold (megawatts) | Annual average curtailment rate (%) | Every year, the lost power generation amount (ten thousand degrees) | Expected annual revenue loss (ten thousand US dollars) |
1st year | 850 | 1200 | 1.2 | 210 | 10.5 |
3rd year | 1400 | 1200 | 4.5 | 788 | 39.4 |
5th year | 2200 | 1500 | 8.2 | 1435 | 71.75 |
7th year | 2800 | 1500 | 12.6 | 2205 | 110.25 |
10th year | 3500 | 2000 | 15.4 | 2695 | 134.75 |
Match large cell
To deal with exceeding 5%'s curtailment rate as well as satisfy power grid peak shaving instructions, alternating current side coupled cell energy storage system (BESS) becomes standard configured module. Configuration proportion normally according to photovoltaic nominal installed capacity's 20% set, namely 100 megawatts photovoltaic matching build 20 megawatts's power conversion system (PCS), cell cabin capacity reaches 40 megawatt-hours (MWh), continuous discharge time satisfies 2 hours standard.
l Cell pack adopts 1500 volts (V) liquid cooling lithium iron phosphate (LFP) scheme, system cycle lifespan guaranteed at every day 1 time full charge full discharge frequency under, running 6000 times after capacity retention rate higher than 70%.
l Cell cluster level's temperature difference needs to be controlled at 3 degrees Celsius within, putting system overall's round trip efficiency (RTE) from air cooling era's 85% pushed high to 89.5%.
l Energy storage container size selects standard 20-foot cabinet, internal stacking density reaches per square meter 250 kilowatt-hours (kWh), single cabinet weight soars to 35 tons.
l Energy management system (EMS) receiving power grid automatic generation control (AGC)'s scheduling instruction after, PCS's power response time needs to be smaller than 200 milliseconds, power control precision error keeps at rated value's 1% within, using this exchanging power grid operator in frequency regulation auxiliary service market inside paying's per megawatt-hour (MWh) 12 US dollars to 18 US dollars's capacity compensation fee.
System Design
Pick large module
Single piece module's rated peak power (Pmax) needs to reach 650 watts (W) to 700 watts (W) interval, standard test condition (STC) under's photoelectric conversion efficiency median value keeps at 22.5% to 23.2%. Physical size's standardization normally is length 2,384 millimeters, width 1303 millimeters, adopting 2 millimeters plus 2 millimeters's semi-tempered glass encapsulation, single piece weight reaches 34.5 kilograms.
Encapsulation film selects co-extruded type polyolefin elastomer (POE), water vapor transmission rate test data needs to be smaller than every day per square meter 1.0 grams, volume resistivity greater than 10 to the 15th power ohm·centimeters, to suppress system voltage at 1500 volts (V) high voltage under induced's potential induced degradation (PID) phenomenon. N-type cell's temperature coefficient distributes in per degree Celsius negative 0.29% to negative 0.31%, when module surface running temperature reaches 65 degrees Celsius, power output's derating amplitude compared to traditional P-type module reduces about 2.5%.
First year light-induced degradation (LID) rate's test upper limit needs to be controlled at 1.0% within, subsequent 24 years' annual linear degradation rate standard deviation must not exceed 0.4%. Bifaciality parameter test sample needs to reach 80% to 85% standard, on the back side receiving per square meter 135 watts (W) diffuse irradiance time, single piece module's actual output current will increase about 1.5 amperes (A) to 2.2 amperes (A).
The past five years' sampling statistics show that photovoltaic module surface's local hot spot effect temperature if continuously 2 hours breaks through 105 degrees Celsius, backplane's insulation layer will happen irreversible thermal degradation, leading to this string's overall power generation amount appearing 4% to 6% negative deviation. Therefore, the infrared thermal imaging drone's whole site inspection frequency needs to be kept at least once every quarter at least 1 time.
Calculate DC/AC ratio
In a high sunshine region with annual global horizontal irradiance (GHI) reaching 1800 kilowatt-hours/square meter (kWh/m²), the DC/AC ratio median is normally set at 1.20 to 1.25. In sunshine conditions, ordinary, and cloudy weather, the proportion exceeding 35% region, the DC/AC ratio's highest value needs to be pushed high to 1.35 to 1.40.
High DC/AC ratio will let the inverter at noon time period trigger clipping running, the inverter will actively deviate from the maximum power point (MPP), leading to about 1.5% to 2.5% absolute energy retention loss. Selecting a rated output power of 350 kilowatts (kW), the string inverter, the whole machine weight is about 115 kilograms, configuring 12 routes to 15 routes independent maximum power point tracking (MPPT) channels, every route channel supported's maximum short circuit current input reaches 40 amperes (A) to 45 amperes (A).
Inverter's startup voltage set at 500 volts (V), when the direct current side string's working voltage exceeds 1500 volts (V)'s physical limit value time, equipment hardware's overvoltage protection relay will in 50 milliseconds inside cut off circuit. Inverter's internal insulated gate bipolar transistor (IGBT) module's switching frequency set at 16 kilohertz (kHz) to 20 kilohertz (kHz), full load running time's shell heat sink temperature peak value needs to be controlled at 75 degrees Celsius below, cooling fan's rotation speed according to internal thermistor's feedback signal at per minute 1200 revolutions to 3500 revolutions between conducting variable frequency adjustment.
Inverter clipping brings energy loss and alternating current side transformer and cable downgrading saved's equipment procurement budget, and there is an obvious negative correlation between clipping loss rate at 2.0% and per megawatt's alternating current side cable procurement cost median value, which is able to drop about 18,000 US dollars, internal rate of return (IRR) instead can obtain 0.3% positive elevation.
Arrange array
Ground coverage ratio (GCR) parameter reflected module projected area and land total area's mathematical ratio. Ground power station's GCR normal distribution is between 0.32 to 0.42. Adopting single-axis tracking bracket system's array planning inside, south-north direction's front back row bracket center spacing (Pitch) normally keeps at 6.0 meters to 7.5 meters.
Selecting 1P (single row portrait) structure's tracking system, single set transmission main shaft's physical length reaches 90 meters, carrying 110 pieces to 120 pieces large size modules, by one unit power being 0.37 kilowatts (kW) or 0.55 kilowatts (kW) direct current brushless motor driven. Transmission shaft's maximum rotation angle range is plus minus 60 degrees, tracking precision's system static error needs to be smaller than 2 degrees. Control board integrated's backtracking algorithm at after sunrise and before sunset's low sun elevation angle time period intervenes, automatically laying flat module tilt angle, making adjacent two rows's shadow length keep at module bottom edge below 100 millimeters place, completely avoiding array between's mutual shading.
Bracket bearing modules need to pass IP65 dustproof waterproof level test, lubricating grease's tolerant temperature range set at minus 40 degrees Celsius to above zero 85 degrees Celsius. Meteorological station's anemometer monitoring frequency is per second 10 times, when instantaneous wind speed breaks through 18 meters/second's threshold, programmable logic controller (PLC) needs in 3 minutes inside putting entire array driven to 0 degrees or 30 degrees's wind protection tilt angle (Stow Position), to lower module surface bearing's lift and drag.
Terrain undulation's dispersion degree will extremely largely affect motor torque's output power, when tracking main shaft at east-west direction's terrain slope drop reaches 10%, motor at climbing rotating process inside's working current peak value will increase 35%, long-term high load running will make control box inside relay's electrical lifespan from 100,000 times cycles sudden drop to less than 30,000 times.
Run cables
From the photovoltaic string's connector end to the direct current combiner box's physical distance, normally between 10 meters to 80 meters, design specification requires the direct current side's voltage drop amplitude to be the highest value limited at 1.5%. Single piece string adopts cross-sectional area being 4 square millimeters or 6 square millimeters's tinned copper core photovoltaic special cable, outer layer cross-linked polyethylene (XLPE) insulation sleeve's anti-ultraviolet (UV) aging lifespan needs to exceed 25 years. Combiner box to inverter's direct current trunk cable selects cross-sectional area in 120 square millimeters to 185 square millimeters's aluminum alloy conductor, current carrying capacity's safety margin coefficient set as 1.25 times.
Buried alternating current cable's trench excavation operation inside, trunk line's buried depth reaches 0.8 meters to 1.2 meters, trench bottom paving thickness being 100 millimeters's fine sand layer acting as buffer. Laying 33 kilovolts (kV) medium voltage cross-linked cable, the bending radius parameter must be greater than the cable outer diameter's 15 times, and the mechanical traction force's pulling strength must not exceed the cable conductor cross-section per square millimeter 40 newtons (N)'s physical limit. Cable joint manufacturing process inside's crimping tube contact resistance needs to be smaller than same length conductor resistance's 1.2 times, partial discharge (PD)'s test reading at 1.5 times rated phase voltage (U0) under must be smaller than 10 picocoulombs (pC).