<PAGE>

                                                                   EXHIBIT 10.15

                                   AGREEMENT

    THIS AGREEMENT is made as of March ___, 2001 by and among MEDWAY ASSOCIATES
LIMITED PARTNERSHIP, a Connecticut limited partnership with an address of c/o
FIP Corporation, 10 McKee Place, Cheshire, Connecticut 06410 ("Medway"), WE
WALLINGFORD LAND, L.L.C., a Delaware limited liability company with an address
of c/o Winstanley Enterprises, LLC, 150 Baker Street Extension, Suite 303,
Concord, Massachusetts 01742 ("WE Wallingford"), and PROTON ENERGY SYSTEMS,
INC., a Delaware corporation with an address of 50 Inwood Road, Rocky Hill,
Connecticut 06070 ("Proton").

                                  BACKGROUND:

     A.   Medway and Winstanley Enterprises, LLC ("Winstanley") entered into a
Contract of Sale dated as of November 1, 2000 (as amended, the "Medway
Contract") for purchase and sale of, in part, land located on Research Parkway
in Wallingford Connecticut and known as Lot W-5A, as more particularly described
on Schedule A attached hereto (the "Entire Parcel").  The Medway Contract was
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assigned to WE Wallingford by Winstanley pursuant to an Assignment dated
February 23, 2001.

     B.   WE Wallingford and Proton have entered or are about to enter into a
Purchase and Sale Agreement (the "Agreement") for the purchase and sale of
approximately 44 acres of the Entire Parcel (the "Property"), as more
particularly described in the Agreement, that is contingent, in part, on the
closing of the sale of the Entire Parcel from Medway to WE Wallingford pursuant
to the Medway Contract.

     C.   In the event WE Wallingford does not take title to the Property on or
before the Closing Date (as defined in the Agreement, the "Closing Date") Medway
has agreed to sell the Property directly to Proton under the terms of the
Agreement.

     NOW THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto hereby agree as
follows:

1.   In the event WE Wallingford does not take title to the Property on or
before the Closing Date, then, upon written notice from Proton given to WE
Wallingford and Medway not later than ten (10) days after the earlier to occur
of (i) Proton's receipt of notice from WE Wallingford that the Medway Contract
has terminated or that WE Wallingford will not be acquiring title to the Entire
Parcel or (ii) the scheduled Closing Date, that Proton desires to purchase the
Property directly from Medway, WE Wallingford's rights and obligations under the
Agreement shall be deemed to be assigned to Medway together with and including
all of WE Wallingford's rights in and to the Deposit (as defined in the
Agreement) and Medway shall assume all obligations of WE Wallingford under the
Agreement, which Agreement will then continue in full force and effect as a
contract for purchase and sale of the Property between Medway and Proton.
2.
3.   Upon assignment of the Agreement to Medway from WE Wallingford, WE
Wallingford shall be forever released and discharged from any and all
responsibility or liability with respect to the Property and its performance
under the Agreement.
4.
5.   During the period of the Agreement that Medway holds title to the
Property, Medway consents to (i) Proton's entry upon the Property for purposes
of its conduct of its due diligence investigations and (ii)
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Proton's and WE Wallingford's application for Approvals (as defined in the
Agreement) pursuant to the terms of the Agreement. During the period that Medway
holds title to the Property, Proton agrees that (i) it shall name Medway and FIP
Realty Corporation, its general partner, ("FIP") as an additional insured on the
insurance certificate required to be delivered by Proton under the provisions of
Section 4(b)(ii) of the Agreement and (ii) its obligation to indemnify and hold
the "Seller" harmless contained in Section 4(i) of the Agreement shall run to
and include Medway and FIP.
6.
7.   During the period of the Agreement that Medway holds title to the Property,
Medway, WE Wallingford and Proton agree to consult with each other and keep the
others generally advised of the progress being made with respect to the
approvals being sought by WE Wallingford and Proton under the provisions of
Section 4(m) and 4(n) of the Agreement (the "Approvals"). In addition to the
consent obligations set forth in the Agreement, Proton and WE Wallingford agree
to obtain Medway's consent and approval (which shall not be unreasonably
withheld) of the applications and all other materials filed in connection with
such Approvals. Medway agrees to, at no expense to Medway, cooperate with WE
Wallingford and Proton in connection with the Approvals being sought by each
and, to the extent necessary, sign applications for Approvals that have been
approved by Medway, in its capacity as the fee owner of the Entire Parcel. WE
Wallingford and Proton will provide to Medway copies of the filings each is
making. During the period that Medway holds title to the Property, Proton shall,
when giving notice to Seller under the provisions of Section 4(b) of the
Agreement give simultaneous notice to Medway. Medway's telephone number and
facsimile number for such notification purpose is (203) 271-0353 (telephone) and
(203) 272-5073 (facsimile), Attention: Mr. David Shopis. The obligations of
notice, communication and consent under this paragraph shall cease at closing of
the Medway Contract.
8.
9.   This agreement may be signed in counterparts, each of which shall be deemed
an original, and all of which together shall constitute one instrument.
10.
11.
12.

      (Rest of page intentionally left blank, signature pages to follow)
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     IN WITNESS WHEREOF, the parties hereto have caused this instrument to be
executed as of the date first above written.

Witnessed:                         MEDWAY ASSOCIATES LIMITED PARTNERSHIP

                                   By: FIP Realty Corporation
______________________________         Its General Partner


______________________________         By:  ________________________________
                                            Name:
                                            Title:

                                   WE WALLINGFORD LAND, L.L.C.

                                   By: Winstanley Enterprises, LLC
______________________________         Its Managing Member


______________________________         By:  _______________________________
                                            Name:
                                            Title:

                                   PROTON ENERGY SYSTEMS, INC.
______________________________


______________________________     By: _______________________________
                                       Name:
                                       Title:


STATE OF CONNECTICUT
                             )     ss.
                             )
COUNTY OF _________________  )

        The foregoing instrument was acknowledged before me this ____ day of
February, 2001 by _______________________ as ______________________ of FIP
Realty Corporation, a Connecticut corporation, partner on behalf of Medway
Associates Limited Partnership, a partnership.

                    __________________________________

                    Commissioner of the Superior Court
                    Notary Public
                    My Commission Expires: __________
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COMMONWEALTH OF MASSACHUSETTS
                                 )  ss.
                                 )
COUNTY OF ______________________ )

        The foregoing instrument was acknowledged before me this ____ day of
February, 2001 by _______________________ as Manager of Winstanley Enterprises,
LLC, a Delaware limited liability company, Managing Member of WE Wallingford
Land, L.L.C., a Delaware limited liability company, on behalf of the limited
liability company.

                    __________________________________

                    Commissioner of the Superior Court
                    Notary Public
                    My Commission Expires: __________


STATE OF CONNECTICUT
                            ) ss.
                            )
COUNTY OF _________________ )

        The foregoing instrument was acknowledged before me this ____ day of
February, 2001 by _______________________ as ______________________ of Proton
Energy Systems, Inc., a _______________________ corporation, on behalf of the
corporation.

                    __________________________________

                    Commissioner of the Superior Court
                    Notary Public
                    My Commission Expires: __________

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