Exhibit 10.91

ENVIRONMENTAL POWER CORPORATION

Bonus Unit Award Agreement

under the 2005 Equity Incentive Plan

This Bonus Unit Award Agreement (the “Agreement”) has been made as of
December 30, 2005, (the “Grant Date”) between Environmental Power Corporation, a
Delaware corporation (the “Company”), and Donald A. Livingston (the
“Participant”).

RECITALS

A. Under the Company’s 2005 Equity Incentive Plan (the “Plan”), the Compensation
Committee (the “Committee”) of the Company’s Board of Directors (the “Board”),
to which Committee authority has been delegated by the Board in accordance with
Section 3(b) of the Plan, has made the award evidenced by this Agreement (the
“Award”) pursuant to Section 7 of the Plan. Capitalized terms not otherwise
defined in this Agreement shall have the respective meanings ascribed to them in
the Plan.

B. The Committee has determined the form of this Agreement and selected the
Participant to receive this Award.

AWARD

In accordance with the Plan, the Company has made this Award, effective as of
the Grant Date and upon the following terms and conditions:

1. Number and Nature of Bonus Units. The number of Bonus Units subject to this
Award is Seven Thousand (7,000). A “Bonus Unit” is a unit of value, valued by
reference to the increase, if any in the Fair Market Value of the Common Stock
over the Initial Value. The “Initial Value” of each Bonus Unit shall be Seven
Dollars ($7.00) per Unit. Each Bonus Unit represents the right, subject to the
terms of the Plan and this Agreement, to receive a payment, in cash or in shares
of Common Stock, at the discretion of the Committee, on the Bonus Date, equal to
the positive difference, if any, between the Initial Value and the Fair Market
Value of a share of the Common Stock on June 30, 2006 (the “Bonus Date”). Bonus
Units are used solely as units of measurement and are not shares of Common
Stock, and the Participant is not, and has no rights as, a shareholder of the
Company by virtue of this Award.

2. Vesting of Bonus Units.

(a) The Bonus Units subject to this Award shall vest, if at all, only if the
Board or the Committee, as the case may be, determines, in its sole discretion,
that the Microgy, Inc. (“Microgy”) pipeline gas development team (“PGD-Team”)
(comprised of the individuals receiving Awards of Bonus Units on the Grant
Date), has made a significant contribution to the advancement of the business of
the Company and Microgy. Without limiting the foregoing, the Board has tasked
the PGD-Team with the goal of achieving the substantial completion, on or before
the Bonus Date, of the work necessary to commence and/or further construction on
at

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least three (3) projects, as well as having one or more additional projects in
development (the “Projects”), consisting of standard Microgy multi-tank
anaerobic digester systems substantially similar to those currently being
developed for the Huckabay project near Stephenville, Texas (the “Huckabay
Project”), or such other projects as otherwise have been approved by the Board
or the appropriate committee thereof for further commitment of Company
resources. The target of (3) projects is inclusive of the Huckabay Project.

(b) In the event that Microgy or the Company is acquired by or merged into, or
all or substantially all of their assets or a majority of their outstanding
voting securities are acquired by, another entity, the Bonus Date shall be
deemed to be the date of such event, the Bonus Units shall be deemed to be
immediately vested as of such date, and any payment in respect thereof shall
immediately become due and payable to the Participant without any further action
on the part of the Board or the Committee.

3. Forfeiture/Expiration. Each Bonus Unit subject to this Award, and any payment
in respect thereof, shall be forfeited, and this Award shall expire and shall be
of no further force or effect, upon the Participant ceasing to continuously
remain an employee of, or consultant or advisor to, the Company or Microgy, or a
member of the PGD-Team (unless the Participant has been reassigned by management
of the Company or Microgy for reasons unrelated to the Participant’s performance
as a member of such team) from the Grant Date through and including the Bonus
Date, except to the extent otherwise provided in Section 2.

4. Payment of Bonus Units. Payment, if any, in respect of Bonus Units subject to
this Award shall be made to the Participant within 30 days of the date that such
Units become vested in accordance with Section 2.

5. No Right to Continued Employment. Nothing in this Agreement or in the Plan
shall confer upon the Participant the right to continue as an employee of, or
consultant or advisor to, the Company or Microgy or as a member of the PGD-Team.

6. Award Not Transferable. The Bonus Units subject to this Award are not
assignable or transferable by the Participant, either voluntarily or by
operation of law, except by will or the laws of descent and distribution. Upon
any attempt to transfer, assign, pledge, hypothecate, sell or otherwise dispose
of any such Bonus Unit, or of any right or privilege conferred hereby, or upon
the levy of any attachment or similar process upon such Bonus Unit, or upon such
right or privilege, such Bonus Unit shall immediately become null and void.

7. Determinations. Determinations by the Board or the Committee, shall be final
and conclusive with respect to the interpretation of the Plan and this
Agreement.

8. Governing Law. The validity and construction of this Agreement shall be
governed by the laws of the state of Delaware applicable to transactions taking
place entirely within that state.

9. Conflicts with Plan and Correction of Errors. In the event that any provision
of this Agreement conflicts in any way with a provision of the Plan, such Plan
provision shall be

 

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controlling and the applicable provision of this Agreement shall be without
force and effect to the extent necessary to cause such Plan provision to be
controlling. In the event that, due to administrative error, this Agreement does
not accurately reflect a Bonus Unit properly granted to Participant pursuant to
the Plan, the Company, acting through its Compensation Committee, reserves the
right to cancel any erroneous document and, if appropriate, to replace the
cancelled document with a corrected document. It is the intention of the Company
and the Participant that this Award not result in unfavorable tax consequences
to Participant under Code Section 409A. Accordingly, Participant consents to
such amendment of this Agreement as the Company may reasonably make in
furtherance of such intention, and the Company shall promptly provide, or make
available to, Participant a copy of any such amendment.

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IN WITNESS WHEREOF, the Company has caused this Agreement to be executed and
granted in Porstmouth, New Hampshire, to be effective as of the Grant Date.

 

ENVIRONMENTAL POWER CORPORATION By:  

/s/ John F. O’Neill

  John F. O’Neill   Chief Executive Officer

PARTICIPANT’S ACCEPTANCE

IN WITNESS OF Participant’s acceptance of this Award and Participant’s agreement
to be bound by the provisions of this Agreement and the Plan, Participant has
signed this Agreement as of December 30, 2005.

 

/s/ Donald A. Livingston

Participant’s Signature

Donald A. Livingston

(print name) Address:

 

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