THERMODYNAMIC MODELING, ENERGY EQUIPARTITION, AND NONCONSERVATION OF ENTROPY FOR DISCRETE-TIME DYNAMICAL SYSTEMS

(1)

AND NONCONSERVATION OF ENTROPY FOR DISCRETE-TIME DYNAMICAL SYSTEMS

WASSIM M. HADDAD, QING HUI, SERGEY G. NERSESOV, AND VIJAYSEKHAR CHELLABOINA

Received 19 November 2004

We develop thermodynamic models for discrete-time large-scale dynamical systems.

Specifically, using compartmental dynamical system theory, we develop energy flow models possessing energy conservation, energy equipartition, temperature equipartition, and entropy nonconservation principles for discrete-time, large-scale dynamical systems. Fur- thermore, we introduce anewand dual notion to entropy; namely,ectropy, as a measure of the tendency of a dynamical system to do useful work and grow more organized, and show that conservation of energy in an isolated thermodynamic system necessarily leads to nonconservation of ectropy and entropy. In addition, using the system ectropy as a Lya- punov function candidate, we show that our discrete-time, large-scale thermodynamic energy flow model has convergent trajectories to Lyapunov stable equilibria determined by the system initial subsystem energies.

1. Introduction

Thermodynamic principles have been repeatedly used in continuous-time dynamical system theory as well as in information theory for developing models that capture the exchange of nonnegative quantities (e.g., mass and energy) between coupled subsystems [5,6,8,11,20,23,24]. In particular, conservation laws (e.g., mass and energy) are used to capture the exchange of material between coupled macroscopic subsystems known as compartments. Each compartment is assumed to be kinetically homogeneous; that is, any material entering the compartment is instantaneously mixed with the material in the compartment. These models are known ascompartmentalmodels and are widespread in engineering systems as well as in biological and ecological sciences [1,7,9,16,17,22].

Even though the compartmental models developed in the literature are based on the first law of thermodynamics involving conservation of energy principles, they do not tell us whether any particular process can actually occur; that is, they do not address the second law of thermodynamics involving entropy notions in the energy flow between subsystems.

The goal of the present paper is directed towards developing nonlinear discrete-time compartmental models that are consistent with thermodynamic principles. Specifically,

(2)

since thermodynamic models are concerned with energy flow among subsystems, we develop a nonlinear compartmental dynamical system model that is characterized by energy conservation laws capturing the exchange of energy between coupled macroscopic subsystems. Furthermore, using graph-theoretic notions, we state three thermodynamic axioms consistent with the zeroth and second laws of thermodynamics that ensure that our large-scale dynamical system model gives rise to a thermodynamically consistent energy flow model. Specifically, using a large-scale dynamical systems theory perspective, we show that our compartmental dynamical system model leads to a precise formulation of the equivalence between work energy and heat in a large-scale dynamical system.

Next, we give a deterministic definition of entropy for a large-scale dynamical system that is consistent with the classical thermodynamic definition of entropy and show that it satisfies a Clausius-type inequality leading to the law of entropy nonconservation.

Furthermore, we introduce anewand dual notion to entropy; namely,ectropy, as a mea- sure of the tendency of a large-scale dynamical system to do useful work and grow more organized, and show that conservation of energy in an isolated thermodynamically consistent system necessarily leads to nonconservation of ectropy and entropy. Then, using the system ectropy as a Lyapunov function candidate, we show that our thermodynamically consistent large-scale nonlinear dynamical system model possesses a continuum of equilibria and issemistable; that is, it has convergent subsystem energies to Lyapunov stable energy equilibria determined by the large-scale system initial subsystem energies. In addition, we show that the steady-state distribution of the large-scale system energies is uniform leading to system energy equipartitioning corresponding to a minimum ectropy and a maximum entropy equilibrium state. In the case where the subsystem energies are proportional to subsystem temperatures, we show that our dynamical system model leads to temperature equipartition, wherein all the system energy is transferred into heat at a uniform temperature. Furthermore, we show that our system-theoretic definition of entropy and the newly proposed notion of ectropy are consistent with Boltzmann’s kinetic theory of gases involving ann-body theory of ideal gases divided by diathermal walls.

The contents of the paper are as follows. In Section 2, we establish notation, definitions, and review some basic results on nonnegative and compartmental dynamical systems. InSection 3, we use a large-scale dynamical systems perspective to develop a nonlinear compartmental dynamical system model characterized by energy conservation laws that is consistent with basic thermodynamic principles. Then we turn our attention to stability and convergence. In particular, using the total subsystem energies as a candidate system energy storage function, we show that our thermodynamic system is lossless and hence can deliver to its surroundings all of its stored subsystem energies and can store all of the work done to all of its subsystems. Next, using the system ectropy as a Lyapunov function candidate, we show that the proposed thermodynamic model is semistable with a uniform energy distribution corresponding to a minimum ectropy and a maximum entropy. InSection 4, we generalize the results ofSection 3to the case where the subsystem energies in large-scale dynamical system model are proportional to subsystem temperatures and arrive at temperature equipartition for the proposed thermodynamic model.

(3)

Furthermore, we provide an interpretation of the steady-state expressions for entropy and ectropy that is consistent with kinetic theory. InSection 5, we specialize the results of Section 3to thermodynamic models with linear energy exchange. Finally, we draw con- clusions inSection 6.

2. Mathematical preliminaries

In this section, we introduce notation, several definitions, and some key results needed for developing the main results of this paper. LetRdenote the set of real numbers, letZ+

denote the set of nonnegative integers, letRⁿdenote the set ofn×1 column vectors, let R^m^×ⁿdenote the set ofm×nreal matrices, let (·)^Tdenote transpose, and letInorIdenote then×nidentity matrix. Forv∈R^q, we writev≥≥0 (resp.,v0) to indicate that every component ofvis nonnegative (resp., positive). In this case, we say thatvisnonnegative orpositive, respectively. LetR+^q andR^q+denote the nonnegative and positive orthants of R^q; that is, ifv∈R^q, thenv∈R^q+andv∈R^q+are equivalent, respectively, tov≥≥0 and v0. Finally, we write · for the Euclidean vector norm,᏾(M) andᏺ(M) for the range space and the null space of a matrixM, respectively, spec(M) for the spectrum of the square matrixM, rank(M) for the rank of the matrixM, ind(M) for the index ofM;

that is, min{k∈Z+: rank(M^k)=rank(M^k⁺¹)},M^# for the group generalized inverse of M, where ind(M)≤1,∆E(x(k)) forE(x(k+ 1))−E(x(k)),Ꮾε(α),α∈Rⁿ,ε >0, for the open ball centered atαwith radiusε, andM≥0 (resp.,M >0) to denote the fact that the Hermitian matrixMis nonnegative (resp., positive) definite.

The following definition introduces the notion ofZ-,M-, nonnegative, and compartmental matrices.

Definition 2.1[2,5,12]. LetW∈R^q^×^q.Wis aZ-matrixifW(i,j)≤0,i,j=1,...,q,i=j.

Wis anM-matrix(resp., anonsingularM-matrix) ifWis aZ-matrix and all the principal minors ofWare nonnegative (resp., positive).Wisnonnegative(resp.,positive) ifW(i,j)≥ 0 (resp.,W(i,j)>0),i,j=1,...,q. Finally,W iscompartmentalifW is nonnegative and _q

i=1W(i,j)≤1,j=1,...,q.

In this paper, it is important to distinguish between a square nonnegative (resp., positive) matrix and a nonnegative-definite (resp., positive-definite) matrix.

The following definition introduces the notion of nonnegative functions [12].

Definition 2.2. Letw=[w1,...,w_q]^T:ᐂ→R^q, whereᐂis an open subset ofR^qthat con- tainsR^q+. Thenwisnonnegativeifw_i(z)≥0 for alli=1,...,qandz∈R^q+.

Note that ifw(z)=Wz, whereW∈R^q^×^q, thenw(·) is nonnegative if and only ifWis a nonnegative matrix.

Proposition2.3 [12]. Suppose thatR^q+⊂ᐂ. ThenR^q+is an invariant set with respect to z(k+ 1)=wz(k), z(0)=z0, k∈Z+, (2.1)

wherez0∈R^q+, if and only ifw:ᐂ→R^qis nonnegative.

(4)

The following definition introduces several types of stability for the discrete-time nonnegativedynamical system (2.1).

Definition 2.4. The equilibrium solutionz(k)≡zeof (2.1) isLyapunov stableif, for every ε >0, there exists δ=δ(ε)>0 such that if z0∈Ꮾδ(ze) ∩ R^q+, thenz(k)∈Ꮾε(ze) ∩ R^q+,k∈Z+. The equilibrium solution z(k)≡ze of (2.1) issemistableif it is Lyapunov stable and there existsδ >0 such that ifz0∈Ꮾδ(ze) ∩ R^q+, then lim_k_→∞z(k) exists and corresponds to a Lyapunov stable equilibrium point. The equilibrium solutionz(k)≡ze

of (2.1) isasymptotically stableif it is Lyapunov stable and there existsδ >0 such that if z0∈Ꮾδ(ze) ∩ R^q+, then lim_k_→∞z(k)=ze. Finally, the equilibrium solutionz(k)≡zeof (2.1) isglobally asymptotically stableif the previous statement holds for allz0∈R^q+.

Finally, recall that a matrixW∈R^q^×^qissemistableif and only if lim_k_→∞W^kexists [12], whileWisasymptotically stableif and only if lim_k_→∞W^k=0.

3. Thermodynamic modeling for discrete-time systems

3.1. Conservation of energy and the first law of thermodynamics. The fundamental and unifying concept in the analysis of complex (large-scale) dynamical systems is the concept of energy. The energy of a state of a dynamical system is the measure of its ability to produce changes (motion) in its own system state as well as changes in the system states of its surroundings. These changes occur as a direct consequence of the energy flow between diﬀerent subsystems within the dynamical system. Since heat (energy) is a fundamental concept of thermodynamics involving the capacity of hot bodies (more energetic subsystems) to produce work, thermodynamics is a theory of large-scale dynamical systems [13]. As in thermodynamic systems, dynamical systems can exhibit energy (due to friction) that becomes unavailable to do useful work. This is in turn contributes to an increase in system entropy; a measure of the tendency of a system to lose the ability to do useful work.

To develop discrete-time compartmental models that are consistent with thermodynamic principles, consider the discrete-time large-scale dynamical systemᏳ shown in Figure 3.1 involvingq interconnected subsystems. Let Ei:Z+→R+ denote the energy (and hence a nonnegative quantity) of theith subsystem, letS_i:Z+→Rdenote the external energy supplied to (or extracted from) theith subsystem, letσ_{i j}:R^q+→R+,i=j, i,j=1,...,q, denote the exchange of energy from the jth subsystem to theith subsystem, and letσii:R^q+→R+,i=1,...,q, denote the energy loss from theith subsystem. Anenergy balanceequation for theith subsystem yields

∆Ei(k)= q j=1,j=i

σi j

E(k)−σji

E(k)−σii

E(k)+Si(k), k≥k0, (3.1)

or, equivalently, in vector form,

E(k+ 1)=wE(k)−dE(k)+S(k), k≥k0, (3.2)

(5)

S1

Si

Sj

Sq

Ᏻ1

Ᏻi

Ᏻj

Ᏻq

.. .

σ11(E)

σii(E)

σj j(E)

σqq(E) σi j(E) σji(E)

Figure 3.1. Large-scale dynamical systemᏳ.

where E(k)=[E1(k),...,Eq(k)]^T, S(k)=[S1(k),...,Sq(k)]^T, d(E(k))=[σ11(E(k)),..., σqq(E(k))]^T,k≥k0, andw=[w1,...,wq]^T:R^q+→R^qis such that

wi(E)=Ei+ q j=1,j=i

σi j(E)−σji(E), E∈R^q+. (3.3)

Equation (3.1) yields a conservation of energy equation and implies that the change of energy stored in theith subsystem is equal to the external energy supplied to (or extracted from) theith subsystem plus the energy gained by theith subsystem from all other subsystems due to subsystem coupling minus the energy dissipated from theith subsystem.

Note that (3.2) or, equivalently, (3.1) is a statement reminiscent of thefirst law of thermo- dynamicsfor each of the subsystems, withE_i(·),S_i(·),σ_{i j}(·),i=j, andσ_ii(·),i=1,...,q, playing the role of the ith subsystem internal energy, energy supplied to (or extracted from) theith subsystem, the energy exchange between subsystems due to coupling, and the energy dissipated to the environment, respectively.

To further elucidate that (3.2) is essentially the statement of the principle of the conservation of energy, let the total energy in the discrete-time large-scale dynamical system Ᏻ be given byUe^TE, E∈R^q+, where e^T[1,..., 1], and let the energy received by the discrete-time large-scale dynamical systemᏳ (in forms other than work) over the discrete-time interval{k1,...,k2}be given byQ^k_k²₌_k₁e^T[S(k)−d(E(k))], whereE(k), k≥k0, is the solution to (3.2). Then, premultiplying (3.2) bye^Tand using the fact that e^Tw(E)≡e^TE, it follows that

∆U=Q, (3.4)

(6)

where∆UU(k2)−U(k1) denotes the variation in the total energy of the discrete-time large-scale dynamical systemᏳover the discrete-time interval{k1,...,k2}. This is a statement of the first law of thermodynamics for the discrete-time large-scale dynamical sys- temᏳand gives a precise formulation of the equivalence between variation in system internal energy and heat.

It is important to note that our discrete-time large-scale dynamical system model does not consider work done by the system on the environment nor work done by the environment on the system. Hence,Qcan be interpreted physically as the amount of energy that is received by the system in forms other than work. The extension of addressing work performed by and on the system can be easily handled by including an additional state equation, coupled to the energy balance equation (3.2), involving volume states for each subsystem [13]. Since this slight extension does not alter any of the results of the paper, it is not considered here for simplicity of exposition.

For our large-scale dynamical system modelᏳ, we assume that σi j(E)=0,E∈R^q+, wheneverE_j=0,i,j=1,...,q. This constraint implies that if the energy of the jth sub- system ofᏳis zero, then this subsystem cannot supply any energy to its surroundings nor dissipate energy to the environment. Furthermore, for the remainder of this paper, we assume thatEi≥σii(E)−Si−_q

j=1,j=i[σi j(E)−σji(E)]= −∆Ei,E∈R^q+,S∈R^q,i=1,...,q.

This constraint implies that the energy that can be dissipated, extracted, or exchanged by theith subsystem cannot exceed the current energy in the subsystem. Note that this as- sumption implies thatE(k)≥≥0 for allk≥k0.

Next, premultiplying (3.2) bye^Tand using the fact thate^Tw(E)≡e^TE, it follows that e^TEk1

=e^TEk0

+

k1−1 k=k0

e^TS(k)−

k1−1 k=k0

e^TdE(k), k1≥k0. (3.5) Now, for the discrete-time large-scale dynamical systemᏳ, define the inputu(k)S(k) and the outputy(k)d(E(k)). Hence, it follows from (3.5) that the discrete-time large- scale dynamical systemᏳislossless[23] with respect to theenergy supply rater(u,y)= e^Tu−e^Tyand with theenergy storage functionU(E)e^TE,E∈R^q+. This implies that (see [23] for details)

0≤Ua

E0

=UE0

=Ur

E0

<∞, E0∈R^q+, (3.6) where

Ua

E0

− inf

u(·),K≥k0

K−1 k=k0

e^Tu(k)−e^Ty(k),

Ur

E0

inf

u(·),K≥−k⁰+1 k0−1 k=−K

e^Tu(k)−e^Ty(k),

(3.7)

andE0=E(k0)∈R^q+. SinceUa(E0) is the maximum amount of stored energy which can be extracted from the discrete-time large-scale dynamical systemᏳat any discrete-time instant K, and Ur(E0) is the minimum amount of energy which can be delivered to

(7)

the discrete-time large-scale dynamical systemᏳto transfer it from a state of minimum potentialE(−K)=0 to a given stateE(k0)=E0, it follows from (3.6) that the discrete- time large-scale dynamical systemᏳcan deliver to its surroundings all of its stored subsystem energies and can store all of the work done to all of its subsystems. In the case whereS(k)≡0, it follows from (3.5) and the fact thatσ_ii(E)≥0,E∈R^q+,i=1,...,q, that the zero solutionE(k)≡0 of the discrete-time large-scale dynamical systemᏳwith the energy balance equation (3.2) is Lyapunov stable with Lyapunov functionU(E) corresponding to the total energy in the system.

The next result shows that the large-scale dynamical systemᏳis locally controllable.

Proposition3.1. Consider the discrete-time large-scale dynamical systemᏳwith energy balance equation (3.2). Then for every equilibrium stateEe∈R^q+and everyε >0andT∈ Z+, there exist Se∈R^q,α >0, andT∈ {0,...,T}such that for every E∈R^q+withE− Ee ≤αT, there exists S:{0,...,T} →R^q such thatS(k)−Se ≤ε,k∈ {0,...,T}, and E(k)=Ee+ ((E−Ee)/T)k,k∈ {0,...,T}.

Proof. Note that withSe=d(Ee)−w(Ee) +Ee, the stateEe∈R^q+is an equilibrium state of (3.2). Letθ >0 andT∈Z+, and define

M(θ,T) sup

E∈Ꮾ1(0),k∈{0,...,T}

wEe+kθE−wEe

−dEe+kθE+dEe

−kθE. (3.8) Note that for everyT∈Z+, lim_θ_→0⁺M(θ,T)=0. Next, letε >0 andT∈Z+ be given, and letα >0 be such thatM(α,T) +α≤ε. (The existence of such anα is guaranteed sinceM(α,T)→0 asα→0⁺.) Now, letE∈R^q+be such that E−Ee ≤αT. WithT E−Ee/α ≤T, wherexdenotes the smallest integer greater than or equal tox, and

S(k)= −wE(k)+dE(k)+E(k) + E−Ee

E−Ee/α, k∈ {0,...,T}, (3.9) it follows that

E(k)=Ee+ E−Ee

E−Ee/αk, k∈ {0,...,T}, (3.10) is a solution to (3.2). The result is now immediate by noting thatE(T) =Eand

S(k)−Se≤ w

Ee+ E−Ee

E−Ee/αk

−wEe

−d

Ee+ E−Ee

E−Ee/αk

+dEe

− E−Ee

E−Ee/αk+α

≤M(α,T) +α

≤ε, k∈ {0,...,T}.

(3.11)

It follows fromProposition 3.1that the discrete-time large-scale dynamical systemᏳ with the energy balance equation (3.2) isreachablefrom andcontrollableto the origin in

(8)

R^q+. Recall that the discrete-time large-scale dynamical systemᏳwith the energy balance equation (3.2) is reachable from the origin inR^q+if, for allE0=E(k0)∈R^q+, there exist a finite timeki≤k0and an inputS(k) defined on{ki,...,k0}such that the stateE(k),k≥ki, can be driven fromE(ki)=0 toE(k0)=E0. Alternatively,Ᏻis controllable to the origin in R^q+if, for allE0=E(k0)∈R^q+, there exist a finite timekf≥k0and an inputS(k) defined on {k0,...,kf}such that the stateE(k), k≥k0, can be driven fromE(k0)=E0toE(kf)=0.

We letᐁr denote the set of all admissible bounded energy inputs to the discrete-time large-scale dynamical systemᏳsuch that for anyK≥ −k0, the system energy state can be driven fromE(−K)=0 toE(k0)=E0∈R+^q byS(·)∈ᐁr, and we letᐁc denote the set of all admissible bounded energy inputs to the discrete-time large-scale dynamical systemᏳsuch that for anyK≥k0, the system energy state can be driven fromE(k0)= E0∈R^q+toE(K)=0 byS(·)∈ᐁc. Furthermore, letᐁbe an input space that is a subset of bounded continuousR^q-valued functions onZ. The spacesᐁr,ᐁc, andᐁare assumed to be closed under the shift operator; that is, ifS(·)∈ᐁ(resp.,ᐁcorᐁr), then the function SKdefined bySK(k)=S(k+K) is contained inᐁ(resp.,ᐁcorᐁr) for allK≥0.

3.2. Nonconservation of entropy and the second law of thermodynamics. The nonlinear energy balance equation (3.2) can exhibit a full range of nonlinear behavior including bifurcations, limit cycles, and even chaos. However, a thermodynamically consistent energy flow model should ensure that the evolution of the system energy is diﬀusive (parabolic) in character with convergent subsystem energies. Hence, to ensure a thermodynamically consistent energy flow model, we require the following axioms. For the statement of these axioms, we first recall the following graph-theoretic notions.

Definition 3.2[2]. Adirected graphG(Ꮿ) associated with theconnectivity matrixᏯ∈R^q^×^q hasvertices{1, 2,...,q}and anarcfrom vertexito vertex j,i=j, if and only ifᏯ(j,i)=0.

AgraphG(Ꮿ) associated with the connectivity matrixᏯ∈R^q^×^q is a directed graph for which thearc setis symmetric; that is,Ꮿ=Ꮿ^T. It is said thatG(Ꮿ) isstrongly connected if for any ordered pair of vertices (i,j),i=j, there exists apath(i.e., sequence of arcs) leading fromitoj.

Recall thatᏯ∈R^q^×^qisirreducible; that is, there does not exist a permutation matrix such thatᏯis cogredient to a lower-block triangular matrix, if and only ifG(Ꮿ) is strongly connected (see [2, Theorem 2.7]). Letφi j(E)σi j(E)−σji(E), E∈R^q+, denote the net energy exchange between subsystemsᏳiandᏳjof the discrete-time large-scale dynamical systemᏳ.

Axiom1. For the connectivity matrixᏯ∈R^q^×^qassociated with the large-scale dynamical systemᏳdefined by

Ꮿ(i,j)=



0 ifφ_{i j}(E)≡0,

1 otherwise, i=j,i,j=1,...,q, Ꮿ(i,i)= −

q k=1,k=i

Ꮿ(k,i), i=j,i=1,...,q,

(3.12)

rankᏯ=q−1, and forᏯ(i,j)=1,i=j,φ_{i j}(E)=0if and only ifE_i=E_j.

(9)

Axiom2. Fori,j=1,...,q,(E_i−E_j)φ_{i j}(E)≤0,E∈R+^q.

Axiom3. Fori,j=1,...,q,(∆Ei−∆Ej)/(E_i−E_j)≥ −1,E_i=E_j.

The fact thatφ_{i j}(E)=0 if and only ifE_i=E_j,i=j, implies that subsystemsᏳi and Ᏻj ofᏳareconnected; alternatively,φ_{i j}(E)≡0 implies thatᏳi andᏳj aredisconnected.

Axiom 1implies that if the energies in the connected subsystemsᏳiandᏳjare equal, then energy exchange between these subsystems is not possible. This is a statement consistent with thezeroth law of thermodynamicswhich postulates that temperature equality is a necessary and suﬃcient condition for thermal equilibrium. Furthermore, it follows from the fact thatᏯ=Ꮿ^T and rankᏯ=q−1 that the connectivity matrixᏯis irreducible which implies that for any pair of subsystemsᏳiandᏳj,i=j, ofᏳ, there exists a sequence of connected subsystems ofᏳthat connect Ᏻi andᏳj.Axiom 2implies that energy is exchanged from more energetic subsystems to less energetic subsystems and is consistent with thesecond law of thermodynamicswhich states that heat (energy) must flow in the direction of lower temperatures. Furthermore, note thatφ_{i j}(E)= −φ_ji(E),E∈R^q+,i=j, i,j=1,...,q, which implies conservation of energy between lossless subsystems. With S(k)≡0, Axioms1and2along with the fact thatφi j(E)= −φji(E),E∈R^q+,i=j,i,j= 1,...,q, imply that at a given instant of time, energy can only be transported, stored, or dissipated but not created and the maximum amount of energy that can be transported and/or dissipated from a subsystem cannot exceed the energy in the subsystem. Finally, Axiom 3implies that for any pair of connected subsystemsᏳiandᏳj,i=j, the energy diﬀerence between consecutive time instants is monotonic; that is, [Ei(k+ 1)−E_j(k+ 1)][E_i(k)−E_j(k)]≥0 for allE_i=E_j,k≥k0,i,j=1,...,q.

Next, we establish a Clausius-type inequality for our thermodynamically consistent energy flow model.

Proposition3.3. Consider the discrete-time large-scale dynamical systemᏳwith energy balance equation (3.2) and assume that Axioms1,2, and3 hold. Then for allE0∈R^q+, kf≥k0, andS(·)∈ᐁsuch thatE(kf)=E(k0)=E0,

k^f−1 k=k⁰

q i=1

S_i(k)−σ_iiE(k) c+Ei(k+ 1)

=

kf−1 k=k0

q i=1

Qi(k) c+Ei(k+ 1)^≤0,

(3.13)

wherec >0,Qi(k)Si(k)−σii(E(k)),i=1,...,q, is the amount of net energy (heat) re- ceived by theith subsystem at thekth instant, andE(k),k≥k0, is the solution to (3.2) with initial conditionE(k0)=E0. Furthermore, equality holds in (3.13) if and only if∆Ei(k)=0, i=1,...,q, andEi(k)=Ej(k),i,j=1,...,q,i=j,k∈ {k0,...,kf−1}.

Proof. SinceE(k)≥≥0,k≥k0, andφ_{i j}(E)= −φ_ji(E),E∈R^q+,i=j,i,j=1,...,q, it fol- lows from (3.2), Axioms 2 and 3, and the fact that x/(x+ 1)≤log_e(1 +x), x >−1

(10)

that

k^f−1 k=k⁰

q i=1

Q_i(k) c+Ei(k+ 1)⁼

k^f−1 k=k⁰

q i=1

∆Ei(k)−_q

j=1,j=iφi j E(k) c+Ei(k+ 1)

=

kf−1 k=k0

q i=1

∆Ei(k) c+Ei(k)

1 + ∆Ei(k) c+Ei(k)

−1

−^k

f−1

k=k0

q i=1

q j=1,j=i

φ_{i j}E(k) c+E_i(k+ 1)

≤ q i=1

log_e

c+E_ikf

c+E_ik0

−^k

f−1

k=k0

q i=1

q j=1,j=i

φ_{i j}E(k) c+E_i(k+ 1)

= −

kf−1 k=k0

q−1 i=1

q j=i+1

φi j

E(k) c+Ei(k+ 1)⁻

φi j

E(k) c+Ej(k+ 1)

= −^k

f−1

k=k0

q−1 i=1

q j=i+1

φ_{i j}E(k)E_j(k+ 1)−E_i(k+ 1) c+E_i(k+ 1)c+E_j(k+ 1)

≤0,

(3.14)

which proves (3.13).

Alternatively, equality holds in (3.13) if and only if^k_k^f₌⁻_k¹₀(∆Ei(k)/(c+Ei(k+ 1)))=0, i=1,...,q, andφ_{i j}(E(k))(E_j(k+ 1)−E_i(k+ 1))=0,i,j=1,...,q,i=j,k≥k0. Moreover, _k_f₋₁

k=k0(∆Ei(k)/(c+Ei(k+ 1)))=0 is equivalent to∆Ei(k)=0,i=1,...,q,k∈ {k0,...,kf− 1}. Hence,φ_{i j}(E(k))(E_j(k+ 1)−E_i(k+ 1))=φ_{i j}(E(k))(E_j(k)−E_i(k))=0,i,j=1,...,q, i=j,k≥k0. Thus, it follows from Axioms1,2, and3that equality holds in (3.13) if and only if∆Ei=0,i=1,...,q, andEj=Ei,i,j=1,...,q,i=j.

Inequality (3.13) is analogous to Clausius’ inequality for reversible and irreversible thermodynamics as applied to discrete-time large-scale dynamical systems. It follows fromAxiom 1and (3.2) that for theisolateddiscrete-time large-scale dynamical systemᏳ; that is,S(k)≡0 andd(E(k))≡0, the energy states given byEe=αe,α≥0, correspond to the equilibrium energy states ofᏳ. Thus, we can define anequilibrium processas a process where the trajectory of the discrete-time large-scale dynamical systemᏳstays at the equilibrium point of the isolated systemᏳ. The input that can generate such a trajectory can be given byS(k)=d(E(k)),k≥k0. Alternatively, anonequilibrium processis a process that is not an equilibrium one. Hence, it follows fromAxiom 1that for an equilibrium process,φi j(E(k))≡0,k≥k0,i=j,i,j=1,...,q, and thus, byProposition 3.3and∆Ei=0, i=1,...,q, inequality (3.13) is satisfied as an equality. Alternatively, for a nonequilibrium process, it follows from Axioms1,2, and3that (3.13) is satisfied as a strict inequality.

Next, we give a deterministic definition of entropy for the discrete-time large-scale dynamical system Ᏻ that is consistent with the classical thermodynamic definition of entropy.

(11)

Definition 3.4. For the discrete-time large-scale dynamical systemᏳwith energy balance equation (3.2), a function᏿:R^q+→Rsatisfying

᏿Ek2

≥᏿Ek1

+

k²−1 k=k1

q i=1

S_i(k)−σ_iiE(k)

c+Ei(k+ 1) , (3.15) for anyk2≥k1≥k0andS(·)∈ᐁ, is called theentropyofᏳ.

Next, we show that (3.13) guarantees the existence of an entropy function forᏳ. For this result, define, theavailable entropyof the large-scale dynamical systemᏳby

᏿a E0

− sup

S(·)∈ᐁc,K≥k0

K−1 k=k0

q i=1

Si(k)−σii E(k)

c+Ei(k+ 1) , (3.16) whereE(k0)=E0∈R^q+andE(K)=0, and define therequired entropy supplyof the large- scale dynamical systemᏳby

᏿r

E0

sup

S(_·)_∈ᐁr,K≥−k0+1 k⁰−1 k=−K

q i=1

S_i(k)−σ_iiE(k)

c+E_i(k+ 1) , (3.17) whereE(−K)=0 and E(k0)=E0∈R^q+. Note that the available entropy᏿a(E0) is the minimum amount of scaled heat (entropy) that can be extracted from the large-scale dynamical systemᏳin order to transfer it from an initial stateE(k0)=E0 toE(K)=0.

Alternatively, the required entropy supply᏿r(E0) is the maximum amount of scaled heat (entropy) that can be delivered toᏳto transfer it from the origin to a given initial state E(k0)=E0.

Theorem3.5. Consider the discrete-time large-scale dynamical systemᏳwith energy bal- ance equation (3.2) and assume that Axioms2 and3hold. Then there exists an entropy function forᏳ. Moreover,᏿a(E),E∈R^q+, and᏿r(E),E∈R^q+, are possible entropy functions forᏳwith᏿a(0)=᏿r(0)=0. Finally, all entropy functions᏿(E),E∈R^q+, forᏳsatisfy

᏿r(E)≤᏿(E)−᏿(0)≤᏿a(E), E∈R^q+. (3.18) Proof. Since, byProposition 3.1,Ᏻis controllable to and reachable from the origin inR^q+, it follows from (3.16) and (3.17) that᏿a(E0)<∞,E0∈R^q+, and᏿r(E0)>−∞,E0∈R^q+, respectively. Next, letE0∈R+^qand letS(·)∈ᐁbe such thatE(ki)=E(kf)=0 andE(k0)= E0, whereki≤k0≤kf. In this case, it follows from (3.13) that

kf−1 k=kⁱ

q i=1

Si(k)−σii E(k)

c+Ei(k+ 1) ^≤0, (3.19)

or, equivalently,

k0−1 k=ki

q i=1

Si(k)−σii E(k) c+Ei(k+ 1) ^{≤ −}

kf−1 k=k0

q i=1

Si(k)−σii E(k)

c+Ei(k+ 1) . (3.20)