Exhibit 5.1

2200 Ross Avenue, Suite 2800 • Dallas, Texas 75201-2784

December 20, 2012

GreenHunter Energy, Inc.

1048 Texan Trail

Grapevine, Texas 76051

At your request, we have examined (i) Amendment No. 4 to the Registration Statement on Form S-3 (No. 333-183292) (the “Registration Statement”) of GreenHunter Energy, Inc., a Delaware corporation (the “Company”), filed pursuant to the Securities Act of 1933, as amended (the “Securities Act”), with the Securities and Exchange Commission (the “SEC”) on December 6, 2012, (ii) the related base prospectus (the “Base Prospectus”), which forms a part of and is included in the Registration Statement, and (iii) the related sales agreement prospectus filed with the SEC on December 20, 2012 (the “ATM Prospectus” and, together with the Base Prospectus, the “Prospectus”), in connection with the registration under the Securities Act of $100 million in undesignated securities covered by the Base Prospectus and the offer and sale to the public through MLV & Co. LLC (the “Sales Manager”), acting as sales manager, under the ATM Prospectus of shares (the “Common Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”), and shares (the “Preferred Shares” and, together with the Common Shares, the “Shares”) of the Company’s 10% Series C Cumulative Preferred Stock, par value $0.001 per share and liquidation preference of $25.00 per share (the “Series C Preferred Stock”), which are convertible into shares (the “Conversion Shares”) of the Company’s Common Stock upon the occurrence of certain events. The Shares will be issued and sold from time to time in at the market transactions pursuant to a sales agreement, dated December 11, 2012, entered into between the Company and the Sales Manager and, with respect to the Preferred Shares, pursuant to the Amended and Restated Certificate of Designations, Rights, Number of Shares and Preferences with respect to the Series C Preferred Stock, adopted by the Company’s Board of Directors on April 24, 2012 and filed with the Secretary of State of the State of Delaware on April 24, 2012 (the “Certificate of Designations”), as corrected by the Certificate of Correction filed with the Secretary of State of the State of Delaware on July 25, 2012 (the “Certificate of Correction”).

In connection with this opinion, we have made such investigations of law as we have deemed appropriate and we have examined the Registration Statement, the Prospectus, and originals, or copies certified or otherwise identified to our satisfaction, of the Certificate of Incorporation, as amended, of the Company, the Certificate of Designations, the Certificate of Correction, the Bylaws of the Company, and such other documents, certificates, records and other instruments as we have deemed appropriate for purposes of the opinion set forth herein.

AUSTIN • BEIJING • DALLAS • DENVER • DUBAI • HONG KONG • HOUSTON • LONDON • LOS ANGELES

MINNEAPOLIS • MUNICH • NEW YORK • RIYADH • SAN ANTONIO • ST. LOUIS • WASHINGTON DC

GreenHunter Energy, Inc.

December 20, 2012

Page 2

We have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of the documents submitted to us as originals, the conformity with the originals of all documents submitted to us as certified, facsimile or photostatic copies and the authenticity of the originals of all documents submitted to us as copies. As to all questions of fact material to this opinion that have not been independently established, we have relied upon certificates or comparable documents of officers and representatives of the Company.

Based upon the foregoing, and having due regard for such legal considerations as we deem relevant, we are of the opinion that the Shares have been duly authorized for issuance and, upon the payment for and delivery of the Shares in accordance with the Registration Statement, the Prospectus, the Certificate of Designations and the Certificate of Correction, such Shares will be validly issued, fully paid and nonassessable, and upon the valid conversion of the Preferred Shares in accordance with the Registration Statement, the Prospectus, the Certificate of Designations and the Certificate of Correction, the Conversion Shares will be duly authorized, validly issued, fully paid and nonassessable. We do not by this letter express any other opinion with respect to the Shares, the Conversion Shares, or any other matter.

In rendering this opinion, we have assumed: (i) that, in addition to the 302,183 Preferred Shares sold pursuant to the ATM Prospectus as of the date hereof, (a) no more than 3,235,208 Common Shares will be sold, based on a sale price of $1.81 per share, representing the last reported sale price of the Company’s Common Stock on NYSE MKT on December 19, 2012, and (b) that no more than 297,245 Preferred Shares will be sold, based on a sale price of $19.70 per share, representing the last reported sale price of the Company’s Series C Preferred stock on NYSE MKT on December 19, 2012; and (ii) that no Shares will be sold that would cause the Company to not satisfy the eligibility requirements for use of Form S-3 (including Instruction I.B.6. thereof).

The opinions expressed herein are limited to the applicable provisions of the Delaware Constitution, the Delaware General Corporation Law, and the rules and regulations and reported judicial and regulatory determinations thereunder and we express no opinion with respect to the laws of any other state or jurisdiction. We expressly disclaim any obligation to advise you of any change in law or subsequent legal or factual developments that might affect any matter or opinion set forth herein.

We hereby consent to the use of this opinion as Exhibit 5.1 to the Registration Statement and to the reference to us under the caption “Legal Matters” in the prospectus included in the Registration Statement. In giving such consent, we do not hereby admit that we are acting within the category of persons whose consent is required under Section 7 of the Securities Act or the rules or regulations of the SEC thereunder.