CN103904696B - A kind of capacity configuration optimization method for island/offshore platform independence new forms of energy micro-capacitance sensor - Google Patents

Abstract

The invention belongs to electric power system micro-capacitance sensor field, relate to the capacity configuration most preferably access method of island/ocean platform independent micro-grid system.The present invention includes: by surveying or estimating the load hourly average power obtaining the island/offshore platform whole year; Local annual natural conditions hourly average data are obtained from meteorological department; Calculate the prediction generated output of each generating equipment unit capacity whole year; The quantity of initial all generating equipment and energy storage device; Calculate the gross generation of all generating equipments in this time period, the total electricity consumption of load; Calculate annual load short of electricity rate and energy residual rate.Invention increases the modifying factor that special natural condition is affected system parameters; Increase special geological surrounding to the modifying factor of cost impact; Add power generation with marine energy equipment and diesel engine, make it both can better utilize regenerative resource to provide electric power, in turn enhance the power supply reliability of system.

Description

A kind of capacity configuration optimization method for island/offshore platform independence new forms of energy micro-capacitance sensor

Technical field

The invention belongs to electric power system micro-capacitance sensor field, relate to the capacity configuration most preferably access method of island/ocean platform independent micro-grid system.

Background technology

Island independent micro-grid system is a kind of electric power system being applicable to island away from land or offshore platform, uses multiple distributed new generating equipment and conventional electric power generation equipment to provide electric energy.In system, the capacity of every Distributed-generation equipment needs to select with suiting measures to local conditions according to local Natural resources condition and loading condiction.But due to the fluctuation of natural energy resources and uncertainty, the ability that energy storage device runs continuously with guarantee system must be added.Generally speaking photovoltaic power generation equipment can be comprised in system, wind-driven generator, power generation with marine energy equipment (comprising marine tidal-current energy, wave energy etc.), diesel engine, miniature gas turbine, energy storage device (as storage battery, flywheel energy storage, super capacitor energy-storage etc.), and relevant power-converting device.

The quality that so numerous its capacity of electrification energy storage equipment is chosen directly has influence on economy and the reliability of whole system, so a kind of outstanding Optimal Capacity choosing method is most important.The present invention, just for this factor, takes into full account the characteristic of ocean, island, devises a kind of capacity configuration optimization method for island/offshore platform independence new forms of energy micro-capacitance sensor targetedly.The result that method in application the present invention draws can be used as the foundation of island/offshore platform micro-capacitance sensor master-plan.

In the domestic existing research optimized about independent micro-capacitance sensor distributed power source capacity, with economy, reliability, the quality of power supply etc. for target, choose single index or multi objective is optimized calculating to wind light mutual complementing, wind-light storage complementary power generation system.Patent " the stored energy capacitance choosing method of wind, light, storage micro-grid system " (application number 201010601680.4) proposes a kind of method of the special optimization stored energy capacitance when generating equipment capacity is determined.Patent " a kind of isolated microgrid distributed power source capacity and optimization method of layouting " (application number 20120371966.7) proposes one and uses genetic algorithm, solves the optimized method of independent micro-capacitance sensor capacity.

But the research object of above research be all the independent micro-capacitance sensor of land routine, does not optimize targetedly for ocean, island micro-grid system, its method and result are also and be not suitable for the capacity optimization computation of island microgrid.In addition due to the particularity of natural resources in ocean, the combination of conventional distributed power generation can not give full play to the traditional characteristic of its complementarity and mutual benefit, Novel generating equipment add the selection that is inevitable.

Summary of the invention

The object of the present invention is to provide a kind of calculating more accurately for the capacity configuration optimization method of island/offshore platform independence new forms of energy micro-capacitance sensor.

The object of the present invention is achieved like this:

(1) obtain the load hourly average power of the island/offshore platform whole year, root determines the annual short of electricity rate DPSP of maximum permission _maxenergy residual rate REPG annual with maximum permission _max, the generating equipment type that selected island/offshore platform autonomous power supply system comprises, at least comprises diesel engine, photovoltaic power generation equipment, wind power plant, ocean current generating device, the one in batteries to store energy;

(2) obtain local annual natural conditions hourly average data from meteorological department, data comprise temperature, intensity of illumination, wind speed, ocean current speed;

(3) with reference to the model of generating equipment, the prediction generated output of each generating equipment unit capacity whole year is calculated:

The output power model of photovoltaic power generation equipment:

$I_{\mathrm{pv}}=I_{\mathrm{sc}}\·\{1-C_1[\exp \left(\frac{V_{\mathrm{pv}}-\mathrm{\ΔV}}{C_2\·V_{\mathrm{oc}}}\right)-1\left]\right\}+\mathrm{\ΔI}$

P＝V _pv·I _pv

In formula: I _pVfor photovoltage model best operating point electric current under arbitrary condition, I _scfor photovoltage model short circuit current, V _pvfor photovoltage model optimum operation point voltage under arbitrary condition, V _ocfor photovoltaic battery panel open circuit voltage,

The output power model of wind power plant:

$P_{\mathrm{wt}}\left(v\right)=\left\{\begin{array}{c}a\·v^3-b\·P_R;v_{\mathrm{ci}}\≤v\≤v_r\\ P_R;v_r\≤v\≤v_{\mathrm{co}}\\ 0;\mathrm{otherwise}\end{array}\right.$

$a=\frac{P_R}{{v_r}^3-{v_{c_i}}^3},b=\frac{{v_{c_i}}^3}{{v_r}^3-{v_{c_i}}^3}$

In formula: P _rfor the rated power of blower fan, ν _rfor the rated wind speed of blower fan, for the incision wind speed of blower fan, v _cofor the cut-out wind speed of blower fan;

The output power model of ocean current machine generating equipment:

$P_{\mathrm{oc}}\left(v\right)=\left\{\begin{array}{c}a\·v^3-b\·P_R;v_{\mathrm{ci}}\≤v\≤v_r\\ P_R;v_r\≤v\≤v_{\mathrm{co}}\\ 0;\mathrm{otherwise}\end{array}\right.$

$a=\frac{P_R}{{v_r}^3-{v_{c_i}}^3},b=\frac{{v_{c_i}}^3}{{v_r}^3-{v_{c_i}}^3}$

In formula: P _rfor the rated power of ocean current machine, ν _rfor the nominal flow rate of ocean current machine, for the incision flow velocity of ocean current machine, v _coflow velocity is cut out for ocean current machine;

(4) quantity of initial all generating equipment and energy storage device, initial number can be decided to be reality can minimum value in Application Range;

(5) establish time t=1, characterize section sometime;

(6) calculate the gross generation of all generating equipments in this time period, the total electricity consumption of load, compares the size of the two;

(7) if generating total amount is greater than electricity consumption total amount in this time period, then calculate energy storage device charging and whether reach the energy storage upper limit, if reach the upper limit, calculate the energy residual in this time period, if do not reach the upper limit, calculate the SOC after energy storage device charging;

The computational methods of EPG and charging SOC:

EPG(t)＝E _Gen(t)-[E _L(t) _ηinv+(SOC _max-SOC(t-1))/η _B]

In formula: E _gent gross energy that () produces for renewable energy power generation, E _lt gross energy that () consumes for load, η _invfor the efficiency of inverter, SOC _maxfor the energy storage device SOC upper limit, the SOC value that SOC (t-1) is previous moment energy storage device, η _bfor energy storage device charge efficiency

SOC(t)＝SOC(t-1)·(1-σ)+[E _Gen(t)-E _L(t)/η _inv]·η _B

In formula: SOC (t) is the state-of-charge of storage battery when time t, SOC (t-1) is the state-of-charge of storage battery when time t-1, and σ is self-discharge rate;

(8) if generating total amount is less than electricity consumption total amount in this time period, then calculate energy storage device electric discharge and whether reach energy storage lower limit, if reach lower limit, calculate the energy disappearance DPS in this time period, if do not reach lower limit, calculate the SOC after energy storage device electric discharge,

The computational methods of DPS and electric discharge SOC:

DPS(t)＝E _L(t)-[E _Gen(t)+SOC(t-1)-SOC _min]η _inv

In formula: SOC _minfor energy storage device SOC lower limit:

SOC(t)＝SOC(t-1)·(1-σ)-[E _L(t)/η _inv-E _Gen(t)]；

(9) when t≤8760, return step 6, t is from increasing 1 simultaneously; Otherwise continue to perform following steps;

(10) annual load short of electricity rate (DPSP) and energy residual rate (REPG) is calculated:

$\mathrm{DPSP}=\underset{t=1}{\overset{T}{\mathrm{\Σ}}}\mathrm{DPS}\left(t\right)/\underset{t=1}{\overset{T}{\mathrm{\Σ}}}{E}_L\left(t\right)$

$\mathrm{REPG}=\underset{t=1}{\overset{T}{\mathrm{\Σ}}}\mathrm{EPG}\left(t\right)/\underset{t=1}{\overset{T}{\mathrm{\Σ}}}{E}_L\left(t\right);$

(11) judge whether DPSP and REPG meets constraints, if met simultaneously, record the configuration combination now selected, photovoltaic capacity, fan capacity, trend machine capacity, energy storage device capacity; Otherwise step after continuing to perform, constraints is:

DPSP≤DPSP _max

REPG≤REPG _max；

(12) if the capacity of the quantity of the generating equipment now selected and energy storage device do not reach reality can maximum in Application Range, then quantity is corresponding to increasing 1, returns step 5 and performs; Otherwise step after continuing to perform;

(13) calculate and contrast overall life cycle cost LCC and the levelized cost of energy LUEC of the combination all meeting constraints, selecting the combination of economy the best as required, the final result for optimizing:

LCC＝C _initial+C _o&m+E _energy+R _replace+S _salvage

$\mathrm{LUEC}=\frac{(C_{\mathrm{initial}}+C_{\mathrm{replace}}+C_{O\&M})\×\mathrm{CRF}}{\underset{t=1}{\overset{8760}{\mathrm{\Σ}}}{E}_{\mathrm{gen}}\left(t\right)}$

In formula: C _initialfor the initial cost of system, C _{o & m}for the cost of equipment operation and maintenance, E _energyfor energy cost, annual all fuel costs, R _replacefor the cost of electricity alternative in the lifetime of system phase, S _salvagefor the net value in residual value, one's last year end of the year.

Beneficial effect of the present invention is:

The present invention is on the basis according to existing similar invention, for the special circumstances on island, ocean, adds the modifying factor affected system parameters special natural condition; Increase special geological surrounding to the modifying factor of cost impact; Add power generation with marine energy equipment (tidal current generator, wave energy generator etc.) and diesel engine, make it both can better utilize regenerative resource to provide electric power, in turn enhance the power supply reliability of system.More than improving and innovating to make the mixing micro-capacitance sensor capacity planning for ocean, island calculate more accurately, more practical.

Accompanying drawing explanation

Fig. 1 is the structure chart of a kind of typical island independent distribution formula electric power system.

Fig. 2 is the flow chart realizing a kind of mode that capacity is distributed rationally.

Embodiment

Below in conjunction with accompanying drawing, the invention will be further described.

For above problem, the present invention proposes a kind of micro-capacitance sensor capacity optimization method for the unique geographical environment in ocean.Proposing the distributed power generation model improved for marine environment, selecting the result of economy optimum when meeting constraints.Its result meets ocean feature, more more close to actual conditions.Island microgrid electricity generation system involved in the present invention, comprises photovoltaic cell, wind-driven generator, energy by ocean current generator, wave energy generator, diesel power generation equipment, batteries to store energy equipment, and its relevant power-converting device.Target of the present invention provides a kind of capacity configuration optimization method for island/offshore platform, under the condition of known natural data situation, take economy as target, many kinds of parameters index is constraint, obtain the optimum capacity configuration of above various Blast Furnace Top Gas Recovery Turbine Unit (TRT) and energy storage device, and the indices data such as economy, fail safe under different configuring condition can be obtained.

A kind of embodiment of the island mixing stand alone generating system capacity optimization method that the present invention proposes is as follows:

In step (1), if DPSP _max=0.1, REPG _max=0.5.The generating equipment selected has photovoltaic power generation equipment, wind-driven generator, ocean current generator, lead-acid battery energy storage.

In step (2), obtain the data of the intensity of illumination of the somewhere whole year, wind speed, temperature, ocean current speed

In step (3), consider the particularity of the natural conditions of ocean, propose the illumination of improvement, Wind speed model, and add additional factor in the device model relevant to cost calculation, model is revised, can the cost of installation and use of equipment in ocean in closing to reality more.

1, the correction of intensity of illumination

Globalradiation on photovoltaic battery panel inclined plane be the intensity of illumination sum of the direct projection of this period on inclined plane, sky diffuse reflection and ground return:

G _G,tilt＝G _dir,tilt+G _diff,tilt+G _refl,tilt

Above three parameters are by the intensity of illumination G of this period on horizontal plane _{g, hor}calculate.Wherein, on inclined plane, direct projection is identical with conventional method with the account form of diffuse reflection intensity of illumination:

$G_{\mathrm{diff},\mathrm{tilt}}=\left[G_{\mathrm{diff},\mathrm{hor}}\frac{1+\cos }{2}\right]\·[1+F\×{\sin }^3(\mathrm{\β}/2)]\·[1+F\×{\cos }^2\mathrm{\θ}\·{\cos }^3{\mathrm{\θ}}_z]$

$G_{\mathrm{dir},\mathrm{tilt}}=G_{\mathrm{dir},\mathrm{hor}}\frac{\cos \mathrm{\θ}}{\cos {\mathrm{\θ}}_z}$

Ground return intensity of illumination G on inclined plane _{refl, tilt}with overall illumination intensity G on horizontal plane _{g, hor}relevant.Due to the particularity of ocean condition, marine illumination reflection is comparatively strong, needs to add modifying factor α during calculating in traditional calculating formula:

$G_{\mathrm{refl},\mathrm{tilt}}=\mathrm{\ρ}\frac{G_{G,\mathrm{hor}(1-\cos )}}{2}\·\mathrm{\α}$

Its value is for being greater than 1, and to install ground periphery ocean surface area relevant with photovoltaic apparatus, and sea is more, and its value is larger.

2, the correction of wind speed

The correction of wind speed is the air speed data in order to the wind speed size measured on anemometer height being converted to the firm setting height(from bottom) place of wind power generation wheel.Like this, the energy output calculating wind energy conversion system output is just more accurate, the change of wind speed and setting height(from bottom) have certain relation, general highly larger, air speed value is also larger, and the size of the size of wind speed and the roughness on earth's surface and temperature also has very large relation, in overhead 500 meters of altitude ranges, the change of wind speed in vertical height, facial index formulae discovery can be pressed:

$\frac{v}{v_0}={\left(\frac{H}{H_0}\right)}^{\mathrm{\α}}$

α is earth's surface coefficient of friction, and decided by the stability of air and the roughness on earth's surface, under ocean condition, its value is generally 0.1 ~ 0.13.

3, the correction of atmospheric density

The size of atmospheric density is different along with the change of the temperature of air, the temperature of steam and height above sea level, this revises the atmospheric density of concrete infield accordingly with regard to needing, due to Island topographic structure more complicated, air humidity is also larger, therefore the energy output of the atmospheric density on island to wind-driven generator is influential, so be necessary to revise atmospheric density, being modified to atmospheric density:

$\mathrm{\ρ}={\mathrm{\ρ}}_0\×\frac{1}{1+0.00366t}\×\left(\frac{p-0.378e}{1000}\right)$

When not having moisture measurement, use the Ideal-Gas Equation to solve density, computing formula is as follows:

$\mathrm{\ρ}=\frac{p}{\mathrm{RT}}$

4, the correction of equipment cost

Cost correction herein comprises installation cost and working service cost.In this natural conditions situation in ocean, not only to consider purchase cost and the installation cost of equipment when calculating the equipment cost of electricity generation system, also need, for the special natural environment in ocean, to consider its annual maintenance cost.

In the present invention, for installation cost, because the place to use of equipment is away from land, its cost of transportation is relevant with the distance on distance land and the value of equipment itself.In an installation, consider the feature needing the anti-high wind of waterproof anti-salt sand prevention, equipment all needs through special process (as three anti-paints etc.).

In the present invention, in the economic model for photovoltaic power generation equipment, wind power plant, add system maintenance cost element, and set up its Mathematical Modeling:

C _(O&M)0＝ζC _i0

In step (4), determine that the reality of equipment can select the scope of quantity, photovoltaic power generation equipment is 0-20, and wind power plant is 0-10, and ocean current generating device is 0-10.

In step (5), initial time is set to t=1.

In step (6), calculate whole gross generation of generating equipment and the total electricity consumption of load in this time period.Size relatively.

In step (7) a metallic, the energy storage upper limit of energy storage device is determined by the total capacity of energy storage device.

In step (8), if generating total amount is less than electricity consumption total amount in this time period, then calculates energy storage device electric discharge and whether reach energy storage lower limit.If reach lower limit, calculate energy disappearance (DPS) in this time period, if do not reach lower limit, calculate the SOC after energy storage device electric discharge.

In step (9), as t > 8760, represent that annual data are complete as calculated.

(10) annual load short of electricity rate (DPSP) and energy residual rate (REPG) is calculated:

$\mathrm{DPSP}=\underset{t=1}{\overset{T}{\mathrm{\Σ}}}\mathrm{DPS}\left(t\right)/\underset{t=1}{\overset{T}{\mathrm{\Σ}}}{E}_L\left(t\right)$

$\mathrm{REPG}=\underset{t=1}{\overset{T}{\mathrm{\Σ}}}\mathrm{EPG}\left(t\right)/\underset{t=1}{\overset{T}{\mathrm{\Σ}}}{E}_L\left(t\right);$

(11) judge whether DPSP and REPG meets constraints, if met simultaneously, record configuration combination (photovoltaic capacity, fan capacity, trend machine capacity, energy storage device capacity) now selected; Otherwise step after continuing to perform.Constraints is:

DPSP≤DPSP _max

REPG≤REPG _max

(12) if the capacity of the quantity of the generating equipment now selected and energy storage device do not reach reality can maximum in Application Range, then quantity is corresponding to increasing 1, returns step 5 and performs; Otherwise step after continuing to perform.

(13) calculating and contrast overall life cycle cost (LCC) and the levelized cost of energy (LUEC) of the combination all meeting constraints, selecting the combination of economy the best as required, is the final result optimized.

LCC＝C _initial+C _o&m+E _energy+R _replace+S _salvage

$\mathrm{LUEC}=\frac{(C_{\mathrm{initial}}+C_{\mathrm{replace}}+C_{O\&M})\×\mathrm{CRF}}{\underset{t=1}{\overset{8760}{\mathrm{\Σ}}}{E}_{\mathrm{gen}}\left(t\right)}.$

Claims (1)

1., for a capacity configuration optimization method for island/offshore platform independence new forms of energy micro-capacitance sensor, it is characterized in that:

(1) obtain the load hourly average power of the island/offshore platform whole year, determine maximum permission annual load short of electricity rate DPSP _maxenergy residual rate REPG annual with maximum permission _max, the generating equipment type that selected island/offshore platform autonomous power supply system comprises, at least comprises diesel engine, photovoltaic power generation equipment, wind power plant, ocean current generating device, the one in batteries to store energy;

(2) obtain local annual natural conditions hourly average data from meteorological department, data comprise temperature, intensity of illumination, wind speed, ocean current speed;

(3) with reference to the model of generating equipment, the prediction generated output of each generating equipment unit capacity whole year is calculated:

The output power model of photovoltaic power generation equipment:

$I_{pv}=I_{sc}.\{1-C_1\[\exp \left(\frac{V_{pv}-\mathrm{\Δ}V}{C_2\·V_{oc}}\right)-1\]\}+\mathrm{\Δ}I$

P＝V _pv·I _pv

In formula: I _pVfor photovoltage model best operating point electric current under arbitrary condition, I _scfor photovoltage model short circuit current, V _pvfor photovoltage model optimum operation point voltage under arbitrary condition, V _ocfor photovoltaic battery panel open circuit voltage,

The output power model of wind power plant:

$P_{wt}\left(v\right)=\left\{\begin{array}{cc}a\·v^3-b\·P_R;& v_{ci}\≤v\≤v_r\\ P_R;& v_r\≤v\≤v_{co}\\ 0;& otherwise\end{array}\right.$

$a=\frac{P_R}{{{\mathrm{\ν}}_r}^3-{{\mathrm{\ν}}_{c_i}}^3},b=\frac{{{\mathrm{\ν}}_{c_i}}^3}{{{\mathrm{\ν}}_r}^3-{{\mathrm{\ν}}_{c_i}}^3}$

In formula: P _rfor the rated power of blower fan, ν _rfor the rated wind speed of blower fan, for the incision wind speed of blower fan, v _cofor the cut-out wind speed of blower fan;

The output power model of ocean current machine generating equipment:

$P_{oc}\left(v\right)=\left\{\begin{array}{cc}a\·v^3-b\·P_S;& v_{si}\≤v\≤v_s\\ P_S;& v_s\≤v\≤v_{so}\\ 0;& otherwise\end{array}\right.$

$a=\frac{P_s}{{{\mathrm{\ν}}_s}^3-{{\mathrm{\ν}}_{s_i}}^3},b=\frac{{{\mathrm{\ν}}_{s_i}}^3}{{{\mathrm{\ν}}_s}^3-{{\mathrm{\ν}}_{s_i}}^3}$

In formula: P _sfor the rated power of ocean current machine, ν _sfor the nominal flow rate of ocean current machine, for the incision flow velocity of ocean current machine, v _soflow velocity is cut out for ocean current machine;

(4) quantity of initial all generating equipment and energy storage device, initial number can be decided to be reality can minimum value in Application Range;

(5) establish time t=1, characterize section sometime;

(6) calculate the gross generation of all generating equipments in this time period, the total electricity consumption of load, compares the size of the two;

(7) if generating total amount is greater than electricity consumption total amount in this time period, then calculate energy storage device charging and whether reach the energy storage upper limit, if reach the upper limit, calculate the energy residual EPG in this time period, if do not reach the upper limit, calculate the SOC after energy storage device charging;

The computational methods of EPG and charging SOC:

EPG(t)＝E _Gen(t)-[E _L(t)/η _inv+(SOC _max-SOC(t-1))/η _B]

In formula: E _gent gross energy that () produces for renewable energy power generation, E _lt gross energy that () consumes for load, η _invfor the efficiency of inverter, SOC _maxfor the energy storage device SOC upper limit, the SOC value that SOC (t-1) is previous moment energy storage device, η _bfor energy storage device charge efficiency,

SOC(t)＝SOC(t-1)·(1-σ)+[E _Gen(t)-E _L(t)/η _inv]·η _B

In formula: SOC (t) is the state-of-charge of storage battery when time t, SOC (t-1) is the state-of-charge of storage battery when time t-1, and σ is self-discharge rate;

(8) if generating total amount is less than electricity consumption total amount in this time period, then calculate energy storage device electric discharge and whether reach energy storage lower limit, if reach lower limit, calculate the energy disappearance DPS in this time period, if do not reach lower limit, calculate the SOC after energy storage device electric discharge

The computational methods of DPS and electric discharge SOC:

DPS(t)＝E _L(t)-[E _Gen(t)+SOC(t-1)-SOC _min]η _inv

In formula: SOC _minfor energy storage device SOC lower limit:

SOC(t)＝SOC(t-1)·(1-σ)-[E _L(t)/η _inv-E _Gen(t)]；

(9) when t≤8760, return step 6, t is from increasing 1 simultaneously; Otherwise continue to perform following steps;

(10) annual load short of electricity rate DPSP and energy residual rate REPG is calculated:

$DPSP=\underset{t=1}{\overset{T}{\Σ}}DPS\left(t\right)/\underset{t=1}{\overset{T}{\Σ}}{E}_L\left(t\right)$

$REPG=\underset{t=1}{\overset{T}{\Σ}}EPG\left(t\right)/\underset{t=1}{\overset{T}{\Σ}}{E}_L\left(t\right);$

(11) judge whether DPSP and REPG meets constraints, if met simultaneously, record the configuration combination now selected, configuration combination comprises photovoltaic capacity, fan capacity, trend machine capacity, energy storage device capacity; Otherwise step after continuing to perform, constraints is:

DPSP≤DPSP _max

REPG≤REPG _max；

(12) if the capacity of the quantity of the generating equipment now selected and energy storage device do not reach reality can maximum in Application Range, then quantity is corresponding to increasing 1, returns step 5 and performs; Otherwise step after continuing to perform;

(13) calculate and contrast overall life cycle cost LCC and the levelized cost of energy LUEC of the combination all meeting constraints, selecting the combination of economy the best as required, the final result for optimizing:

LCC＝C _initial+C _o&m+E _energy+R _replace+S _salvage

In formula: C _initialfor the initial cost of system, C _{o & m}for the cost of equipment operation and maintenance, E _energyfor energy cost, annual all fuel costs, R _replacefor the cost of electricity alternative in the lifetime of system phase, S _salvagefor the net value in residual value, one's last year end of the year.