16. Commitments and Contingencies

Operating Leases

As of December 31, 2015 and 2014, the Company has several non-cancelable operating leases (as lessor and as lessee), primarily associated with sale/leaseback transactions that are partially secured by restricted cash (see also note 1) as summarized below and expire over the next six years. Minimum rent payments under operating leases are recognized on a straight-line basis over the term of the lease. Leases where the Company is the lessor contain termination clauses with associated penalties, the amount of which cause the likelihood of cancellation to be remote.

Future minimum lease payments under non-cancelable operating leases (with initial or remaining lease terms in excess of one year) as of December 31, 2015 are (in thousands):

	As Lessor		As Lessee
2016	$	10,411	$	11,773
2017		10,411		11,794
2018		10,411		11,577
2019		10,411		10,386
2020		9,255		9,249
2021 and thereafter		5,022		5,503

Total future minimum lease payments	$	55,921	$	60,282

Rental expense for all operating leases were $6.2 million, $1.5 million, and $769 thousand for years ended December 31, 2015, 2014, and 2013 respectively.

At December 31, 2015 and 2014, prepaid rent and security deposits associated with sale/leaseback transactions were $12.1 million and zero, respectively.

Finance Obligation

During the year ended December 31, 2015, the Company received cash for future services to be performed associated with certain sale/leaseback transactions. This cash received representing amounts related to future services amounted to $15.1 million. The short-term portion of this amount of $2.6 million is included in other current liabilities, with the remaining balance included in finance obligations within the accompanying consolidated balance sheet at December 31, 2015. The amount is amortized using the effective interest method.

In 2013, the Company completed a sale-leaseback transaction of its property in Latham, New York, for an aggregate sale price of $4.5 million. Although the property was sold and the Company has no legal ownership of the facility, the Company was prohibited from recording the transaction as a sale because of continuing involvement with the property. Accordingly, the sale has been accounted for as a financing transaction, which requires the Company to continue reporting the building as an asset and to record a financing obligation for the sale price. Liabilities relating to this agreement of $2.4 million and $74 thousand, $2.4 million and $66 thousand have been recorded as finance obligation and current portion finance obligation (other current liabilities), respectively, in the accompanying consolidated balance sheets as of December 31, 2015 and 2014, respectively .

Restricted Cash

The Company has entered into sale/leaseback agreements associated with its products and services. In connection with these agreements, cash of $46.8 million is required to be restricted as security and will be released over the lease term. The Company has additional letters of credit backed by security deposits as disclosed in the Operating Leases section above.

The Company also has letters of credit in the aggregate amount of $1.0 million associated with an agreement to provide hydrogen infrastructure and hydrogen to a customer at its distribution center and with a finance obligation from the sale/leaseback of its building. Cash collateralizing these letters of credit is considered restricted cash.

Litigation

Legal matters are defended and handled in the ordinary course of business. The Company has established accruals for matters for which management considers a loss to be probable and reasonably estimable. It is the opinion of management that facts known at the present time do not indicate that such litigation, after taking into account insurance coverage and the aforementioned accruals, will have a material adverse impact on our results of operations, financial position, or cash flows.

Concentrations of credit risk

Concentrations of credit risk with respect to receivables exist due to the limited number of select customers with whom the Company has initial commercial sales arrangements. To mitigate credit risk, the Company performs appropriate evaluation of a prospective customer's financial condition.

At December 31, 2015, two customers comprise approximately 50.9% of the total accounts receivable balance, with each customer individually representing 38.5% and 12.4% of total accounts receivable, respectively. At December 31, 2014, four customers comprise approximately 69.9% of the total accounts receivable balance, with each customer individually representing 30.2%, 16.0%, 13.4% and 10.3% of total accounts receivable, respectively.

For the year ended December 31, 2015, 56.7% of total consolidated revenues were associated primarily with Walmart. For the year ended December 31, 2014, 37.2% of total consolidated revenues were associated primarily with Walmart and Volkswagen, with each representing 24.1% and 13.1% of total consolidated revenues, respectively. For the year ended December 31, 2013, 33.2% of our total consolidated revenues were associated Mercedes-Benz, Procter & Gamble, and Lowe's representing 11.6%, 11.2%, and 10.4% of total consolidated revenues, respectively.