How to properly implement a free product offering?

Question

If I'm listing a free online product, is there a recommended way to use the wording "Free" when structuring its price, rather than $0.00?

schema.org/Offer specification says a string representation of a price must be of type PriceSpecification, I'm not sure what it means.

Alternative #1:

<div itemprop="offers" itemscope itemtype="http://schema.org/Offer">
    <span itemprop="price">Free</span>
    <meta itemprop="priceCurrency" content="USD" />
</div>

Alternative #2:

<div itemprop="offers" itemscope itemtype="http://schema.org/Offer">
    <span itemprop="description">Free</span>
    <meta itemprop="price" content="0" />
    <meta itemprop="priceCurrency" content="USD" />
</div>

I'm leaning toward alternative #1 (seems obvious) but I've had no luck finding a reference explicitly stating that use.

Answer 1

The alternative #2 is a correct way for most of semantical analysis performed by the search engines crawlers, like googlebot. With other words, Google will understand that the offer is free with the second alternative.