EXHIBIT
10.23




AMENDMENT
NO. 4





THIS AMENDMENT TO FACILITIES AND
SUPPORT SERVICES AGREEMENTis
effective as of January 1, 2010 between Tecogen Inc., a Delaware corporation
(“Tecogen”),
and American DG Energy Inc., a Delaware corporation (“ADG
Energy”).





NOW,
THEREFORE, for good and valuable consideration, the receipt and sufficiency of
which is hereby acknowledged, the parties hereby reaffirm all of the provisions
of the Facilities and Support Services Agreement, dated  January 1,
2006 (the “Agreement”), except that the following paragraphs shall be
substituted for the corresponding paragraphs in the Agreement.





1.           Office and
Infrastructure





Tecogen
will provide to ADG Energy the following office and infrastructure support
services for a period from the date of this Agreement through the Termination
Date (as such term is defined in Section 2 below):





(a)           Office Space. Approximately
3,339 allocated square feet of space in Tecogen’s offices located at 45 First
Avenue, Waltham, Massachusetts 02451 (the “Building”), which
shall include six (6) offices, a shared conference room and manufacturing space.
Tecogen will also provide ADG Energy with water, sewer, electrical, phones and
other utility services, heating, ventilation and air-conditioning, and cleaning
and janitorial services. Tecogen may change the space in the Building occupied
by ADG Energy from time to time during the term of this Agreement. Tecogen will
provide such space and services at a flat rate of $5,526.00 per month. If
additional space is provided, this flat fee will increase at an annual rate of
$15.84 per square foot for any additional office space and $14.05 per square
foot for any additional manufacturing space. Copy machine usage, office
supplies, and shipping, secretarial & receptionist services, Internet
service, telephone support and IT support are not included in the monthly rate
and will be billed separately.





(c)           Insurance and Employee Benefit
Plans. To the extent it is able to do under its then current plans and
policies, Tecogen will include ADG Energy as a covered entity under its
liability, property and casualty, workers compensation and other applicable
business insurance policies. Tecogen will allow ADG Energy employees to
participate in Tecogen’s medical and dental insurance plans, and other group
insurance plans for its employees, including life, AD&D, and short and
long-term disability plans. The costs of these insurance programs will be
charged to ADG Energy on an actual cost basis when available, or in the case of
general insurance be allocated to ADG Energy for it’s pro rata share of the
premiums. Management of the plans will be carried out jointly by both companies
and with no charge to each other.





(g)           Exclusivity: ADG Energy shall
be granted exclusive representation rights to the Tecogen Cogeneration Product
in the New England States, and Tecogen shall be granted exclusive representation
rights to the ADG Energy Cogeneration Product in California. The relevant
portions of Tecogen’s standard rep agreement shall apply except where in
conflict with this agreement. ADG Energy will be eligible for split commissions
for cogeneration and chiller products as presented in Tecogen’s standard rep
agreement.





2.  Term





The term of the Agreement, as amended
hereby, commenced as of the effective date for one year, renewable annually upon
mutual written agreement.





IN
WITNESS WHEREOF, the parties hereto have caused this amendment to be duly
executed and delivered by their proper and duly authorized representatives as of
the day and year first above written.





Dated:
January 4, 2010 (but effective as set forth above).




TECOGEN
      INC.
              
              
              AMERICAN
      DG ENERGY INC.By:
       /s/ Robert A.
      Panora
                
                Name: Robert A.
      Panora
              
              
              By: 
      /s/ Barry J.
      Sanders
                
                Name: Barry J. SandersTitle: President
              
              
              Title: President