SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
SCHEDULE 13G
Under the Securities Exchange Act of 1934
(Amendment No. __)*
Active Power, Inc.
(Name of Issuer)
Common Stock, par value $0.001 per share
(Title of Class of Securities)
00504W100
(CUSIP Number)
March 7, 2012
(Date of Event Which Requires Filing of This Statement)
Check the appropriate box to designate the rule pursuant to which this Schedule is filed:
¨	Rule 13d-1(b)
x	Rule 13d-1(c)
¨	Rule 13d-1(d)
(Page 1 of 17 Pages)

______________________________

*The remainder of this cover page shall be filled out for a reporting person's initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be "filed" for the purpose of Section 18 of the Securities Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No.  00504W100   13G Page 2 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Partners Fund II, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,951,047 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,951,047 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,951,047 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 2.0%
12	TYPE OF REPORTING PERSON PN

CUSIP No.  00504W100   13G Page 3 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Partners Institutional Fund, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 1,594,300 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 1,594,300 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 1,594,300 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 1.7%
12	TYPE OF REPORTING PERSON PN

CUSIP No.  00504W100   13G Page 4 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Partners Renewable Energy Fund, L.P.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 2,649,236 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 2,649,236 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 2,649,236 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 2.8%
12	TYPE OF REPORTING PERSON PN

CUSIP No.  00504W100   13G Page 5 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Offshore Fund, Ltd.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION British Virgin Islands
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 129,800 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 129,800 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 129,800 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.1%
12	TYPE OF REPORTING PERSON CO

CUSIP No.  00504W100   13G Page 6 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Renewable Energy Offshore Fund, Ltd.
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION British Virgin Islands
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 176,400 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 176,400 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 176,400 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 0.2%
12	TYPE OF REPORTING PERSON CO

CUSIP No.  00504W100   13G Page 7 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Advisory Partners
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION New York
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 7,278,383 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 7,278,383 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,278,383 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%
12	TYPE OF REPORTING PERSON PN; IA

CUSIP No.  00504W100   13G Page 8 of 17 Pages

1	NAME OF REPORTING PERSON Ardsley Partners I
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION New York
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 6,194,583 shares of Common Stock
	6	SHARED VOTING POWER 0
	7	SOLE DISPOSITIVE POWER 6,194,583 shares of Common Stock
	8	SHARED DISPOSITIVE POWER 0
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 6,194,583 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 6.5%
12	TYPE OF REPORTING PERSON PN

CUSIP No.  00504W100   13G Page 9 of 17 Pages

1	NAME OF REPORTING PERSON Philip J. Hempleman
2	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP		(a) x (b) ¨
3	SEC USE ONLY
4	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5	SOLE VOTING POWER 0
	6	SHARED VOTING POWER 7,278,383 shares of Common Stock
	7	SOLE DISPOSITIVE POWER 0
	8	SHARED DISPOSITIVE POWER 7,278,383 shares of Common Stock
9	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,278,383 shares of Common Stock
10	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES			¨
11	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%
12	TYPE OF REPORTING PERSON IN

CUSIP No.  00504W100   13G Page 10 of 17 Pages

Item 1(a).	NAME OF ISSUER
	The name of the issuer is Active Power, Inc. (the "Company").

Item 1(b).	ADDRESS OF ISSUER'S PRINCIPAL EXECUTIVE OFFICES
	The Company's principal executive offices are located 2128 W. Braker Lane, BK12, Austin, Texas 78758.

Item 2(a).	NAME OF PERSON FILING
	This statement is filed by:
	(i)	Ardsley Partners Fund II, L.P., a Delaware limited partnership ("AP II"), with respect to the shares of common stock, par value $0.001 per share ("Common Stock") directly owned by it;
	(ii)	Ardsley Partners Institutional Fund, L.P., a Delaware limited partnership ("Ardsley Institutional"), with respect to the shares of Common Stock directly owned by it;
	(iii)	Ardsley Partners Renewable Energy Fund, L.P., a Delaware limited partnership ("Ardsley Energy"), with respect to the shares of Common Stock directly owned by it;
	(iv)	Ardsley Offshore Fund, Ltd., a British Virgin Islands corporation ("Ardsley Offshore"), with respect to the shares of Common Stock directly owned by it;
	(v)	Ardsley Renewable Energy Offshore Fund, Ltd., a British Virgin Islands Corporation ("Ardsley Energy Offshore"), with respect to the shares of Common Stock directly owned by it;
	(vi)	Ardsley Advisory Partners, a New York general partnership ("Ardsley") which serves as Investment Manager of Ardsley Offshore and Ardsley Energy Offshore and as Investment Adviser of AP II, Ardsley Institutional, Ardsley Energy and a certain managed account, with respect to the shares of Common Stock directly owned by Ardsley Offshore, Ardsley Energy Offshore, AP II, Ardsley Institutional, Ardsley Energy and the managed account;
	(vii)	Ardsley Partners I, a New York general partnership ("Ardsley Partners") which serves as General Partner of AP II, Ardsley Institutional and Ardsley Energy, with respect to the shares of Common Stock owned by AP II, Ardsley Institutional and Ardsley Energy; and
	(viii)	Philip J. Hempleman ("Mr. Hempleman"), the Managing Partner of Ardsley and Ardsley Partners, with respect to the shares of Common Stock owned by AP II, Ardsley Institutional, Ardsley Energy, Ardsley Offshore, Ardsley Energy Offshore and the managed account.
	The foregoing persons are hereinafter sometimes collectively referred to as the "Reporting Persons."  Any disclosures herein with respect to persons other than the Reporting Persons are made on information and belief after making inquiry to the appropriate party.  The filing of this statement should not be construed as an admission that any of the Reporting Persons is, for the purpose of Section 13 of the Act, the beneficial owner of the Common Stock reported herein.

CUSIP No.  00504W100   13G Page 11 of 17 Pages

Item 2(b).	ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IF NONE, RESIDENCE
	The address of the business office of each of the Reporting Persons, with the exception of Ardsley Offshore and Ardsley Energy Offshore, is 262 Harbor Drive, Stamford, Connecticut 06902.
	The address of the registered office of Ardsley Offshore and Ardsley Energy Offshore is Romasco Place, Wickhams Cay 1, Road Town Tortola, British Virgin Islands.

Item 2(c).	CITIZENSHIP
	AP II, Ardsley Institutional and Ardsley Energy are Delaware limited partnerships.  Ardsley Offshore and Ardsley Energy Offshore are British Virgin Islands corporations.  Ardsley and Ardsley Partners are New York general partnerships.  Mr. Hempleman is a United States Citizen.

Item 2(d).	TITLE OF CLASS OF SECURITIES
	Common Stock, par value $0.001 per share.

Item 2(e).	CUSIP NUMBER
	00504W100

CUSIP No.  00504W100   13G Page 12 of 17 Pages

Item 3. IF THIS STATEMENT IS FILED PURSUANT TO Rules 13d-1(b), OR 13d-2(b) OR (c), CHECK WHETHER THE PERSON FILING IS A:

(a)	¨	Broker or dealer registered under Section 15 of the Act;
(b)	¨	Bank as defined in Section 3(a)(6) of the Act;
(c)	¨	Insurance company as defined in Section 3(a)(19) of the Act;
(d)	¨	Investment company registered under Section 8 of the Investment Company Act of 1940;
(e)	¨	An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);
(f)	¨	An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);
(g)	¨	A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);
(h)	¨	A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act;
(i)	¨	A church plan that is excluded from the definition of an investment company under Section 3(c)(14) of the Investment Company Act;
(j)	¨	A non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J);
(k)	¨	Group, in accordance with Rule 13d-1(b)(1)(ii)(K).

If filing as a non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J), please specify the type of institution: __________________________________

Item 4.	OWNERSHIP
	The information required by Items 4(a) - (c) is set forth in Rows 5 - 11 of the cover page for each Reporting Person hereto and is incorporated herein by reference for each Reporting Person.
	The Company communicated to the Reporting Persons via email on March 9, 2012 that the total number of outstanding shares of Common Stock as of March 7, 2012 was 95,427,235.  The percentages used herein and in the rest of this Schedule 13G are based upon such number of shares of Common Stock outstanding determined in accordance with Rule 13-D(1)(j).

Item 5.	OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS
	Not applicable.

CUSIP No.  00504W100   13G Page 13 of 17 Pages

Item 6.

OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON

Ardsley, the Investment Manager of Ardsley Offshore and Ardsley Energy Offshore and the Investment Adviser of AP II, Ardsley Institutional, Ardsley Energy and a certain managed account, has the power to vote and direct the disposition of the proceeds from the sale of the shares of Common Stock owned by Ardsley Offshore, Ardsley Energy Offshore, AP II, Ardsley Institutional, Ardsley Energy and such managed account, and accordingly may be deemed the direct "beneficial owner" of such shares of Common Stock. Ardsley Partners, the General Partner of AP II, Ardsley Institutional and Ardsley Energy, shares the power to vote and direct the disposition of the shares of Common Stock owned by AP II, Ardsley Institutional and Ardsley Energy, and, accordingly, may be deemed the direct "beneficial owner" of such shares of Common Stock. Mr. Hempleman is the Managing Partner of Ardsley and Ardsley Partners and in that capacity directs their operations and therefore may be deemed to be the indirect "beneficial owner" of the shares of Common Stock owned by AP II, Ardsley Offshore, Ardsley Institutional, Ardsley Energy, Ardsley Energy Offshore and a certain managed account.  Mr. Hempleman disclaims beneficial ownership of all of the shares of Common Stock reported in this 13G.

Item 7.	IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY OR CONTROL PERSON
	Not applicable.

Item 8.	IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP
	Not applicable.

Item 9.	NOTICE OF DISSOLUTION OF GROUP
	Not applicable.

Item 10.	CERTIFICATION
	Each of the Reporting Persons hereby makes the following certification:
	By signing below each Reporting Person certifies that, to the best of his or its knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

CUSIP No.  00504W100   13G Page 14 of 17 Pages

SIGNATURES

After reasonable inquiry and to the best of his or its knowledge and belief, each of the undersigned certifies that the information set forth in this statement is true, complete and correct.

DATE:  March 16, 2012

ARDSLEY PARTNERS FUND II, L.P.

BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY PARTNERS INSTITUTIONAL FUND, L.P.
BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY PARTNERS RENEWABLE ENERGY FUND, L.P.
BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY OFFSHORE FUND, LTD.
BY:	/s/ Steve Napoli
	Steve Napoli
	Director
ARDSLEY RENEWABLE ENERGY OFFSHORE FUND, LTD.
BY:	/s/ Steve Napoli
	Steve Napoli
	Director

CUSIP No.  00504W100   13G Page 15 of 17 Pages

		ARDSLEY ADVISORY PARTNERS
		BY:	/s/ Steve Napoli
			Steve Napoli
			Partner
		ARDSLEY PARTNERS I
		BY:	/s/ Steve Napoli
			Steve Napoli
			General Partner
		PHILIP J. HEMPLEMAN, INDIVIDUALLY
		BY:	/s/ Steve Napoli*
			Steve Napoli
			As attorney in fact for
			Philip J. Hempleman
*	Evidence of Power of Attorney was filed with the Schedule 13G/A filed on February 15, 2006 (Acc-no: 0000902664-06-000895) and is incorporated by reference into this filing.

CUSIP No.  00504W100   13G Page 16 of 17 Pages

EXHIBIT 1

JOINT FILING AGREEMENT

PURSUANT TO RULE 13d-1(k)

The undersigned acknowledge and agree that the foregoing statement on Schedule 13G is filed on behalf of each of the undersigned and that all subsequent amendments to this statement on Schedule 13G shall be filed on behalf of each of the undersigned without the necessity of filing additional joint filing agreements.  The undersigned acknowledge that each shall be responsible for the timely filing of such amendments, and for the completeness and accuracy of the information concerning him or it contained herein and therein, but shall not be responsible for the completeness and accuracy of the information concerning the others, except to the extent that he or it knows or has reason to believe that such information is inaccurate.

DATE:  March 16, 2012

ARDSLEY PARTNERS FUND II, L.P.
BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY PARTNERS INSTITUTIONAL FUND, L.P.
BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY PARTNERS RENEWABLE ENERGY FUND, L.P.
BY:	ARDSLEY PARTNERS I,
	GENERAL PARTNER
BY:	/s/ Steve Napoli
	Steve Napoli
	General Partner
ARDSLEY OFFSHORE FUND, LTD.
BY:	/s/ Steve Napoli
	Steve Napoli
	Director

CUSIP No.  00504W100   13G Page 17 of 17 Pages

		ARDSLEY RENEWABLE ENERGY OFFSHORE FUND, LTD.
		BY:	/s/ Steve Napoli
			Steve Napoli
			Director
		ARDSLEY ADVISORY PARTNERS
		BY:	/s/ Steve Napoli
			Steve Napoli
			Partner
		ARDSLEY PARTNERS I
		BY:	/s/ Steve Napoli
			Steve Napoli
			General Partner
		PHILIP J. HEMPLEMAN, INDIVIDUALLY
		BY:	/s/ Steve Napoli*
			Steve Napoli
			As attorney in fact for
			Philip J. Hempleman
*	Evidence of Power of Attorney was filed with the Schedule 13G/A filed on February 15, 2006 (Acc-no: 0000902664-06-000895) and is incorporated by reference into this filing.