Editor's note: The Energy Services Bulletin features real answers to real questions posed to our staff at the Energy Services Power Line. We hope you find it useful.

Hotel owner seeks to capture waste heat

Our hotel and restaurant are currently heated with a 30-year-old, space-heating boiler we expect to replace soon. Also, we use a natural-gas water heater for both facilities. We were wondering if we could use waste soybean oil from the restaurant kitchen as fuel for the water heater and boiler. The restaurant generates about 500 gallons of waste oil annually. Also, we would like to recover the heat from the dishwasher's 130-degree wastewater.

Answer:

You are suggesting some innovative ideas for waste problems typical to the hospitality industry. Choosing a heating source is a complex process that must balance capital cost, expected operating cost, space, need for heat, operating life, environmental restrictions, efficiency, fuel and operating flexibility, reliability and availability and schedule. There are a few options you might consider, all of them compromises:

1. Replace your existing gas boiler with a newer, more energy-efficient one. This is the simplest and probably the least expensive choice. Advances in technology over the last thirty years make these systems very economical.
   
   
2. Install one gas-powered unit boiler for both space- and water-heating systems. Depending on whether your existing boiler produces steam or hot water, a number of distribution system, steam trap, radiator or control measures to improve your heating-system efficiency may be possible.
   
   
3. Install a natural gas-powered generator with a cogeneration system that can produce electricity and preheat water for your facility.
   
   
4. Install a biodiesel generator for generating electricity. Jacket water used to cool the engine can be heated to 140° F or higher.
   
   
5. To heat water using vegetable oil fuel, install any kind of diesel or fuel oil boiler. Biodiesel specialists indicate that there are only a few small biodiesel-powered cogeneration systems in use in North America (for example, wind power and biodiesel generators).

In theory, using waste vegetable oil as a fuel sounds like a good idea. Vegetable oil is an important potential fuel because it comes from plants and is a renewable resource. As a waste product, it would appear to be a "free" fuel.

In practice, heating with waste vegetable oil is likely to be an expensive and complex project. First, used vegetable oil must be converted to biodiesel through a chemical process called transesterification. This is not a difficult process, but it does require space and special equipment, and there is always the danger of spills. 

Although 500 gallons per year sounds like a lot, soybean oil's fuel value in place of natural gas is about 63.8 Mbtu per year (500 Gal./year x 0.92 x 138,700 Btu/Gal.=63,800,000 Btu/year). Assuming a natural gas price at 32 to 50 cents per 100,000 Btu, using waste oil for heat would only save you $204 to $319 per year.

A small diesel generator set produces 20 kW, weighs over one ton, has an electrical efficiency of 27 percent, and probably costs $20,000 installed. Such systems generally run on natural gas, and require a second fuel heating and injection system for liquid or dual-fuel operation. Your investment would only pay off if you had more than 10 times more waste oil and experienced frequent, extended power outages. As long as the cost of natural gas remains quite low, it remains a better value.

Waste heat recovery may be a more cost-effective option than a biodiesel powered boiler. It takes advantage of a no-cost source of energy using simple, low-cost equipment that requires little energy for operation.

Businesses like restaurants and laundries spend a lot to heat water and most of that water gets discharged into the sewer while still very hot. Preheating incoming cold water with wastewater heat could recover as much as 60 percent of that energy. Under ideal conditions, recovering that heat can actually double the effective efficiency of a water heater. In addition, there may be environmental benefits from reducing the temperature of the wastewater stream to avoid possible thermal pollution.

Waste heat recovery water heating is typically uses recovered waste heat to heat incoming cold water. A conventional water heater supplies supplemental heat.

A water heat recovery device called GFX costs between $89 and $450, and increases the efficiency of a water heater by 70 percent by capturing waste heat from drain water and preheating incoming cold water. It saves 30 to 50 percent of total heating energy consumption and also effectively triples the heating capacity of the water heater.

You might not limit heat recovery only to dishwashers or laundry. If wastewater quantity and temperature are high, it could also be applied to showers. The cold water might need to be circulated through a warm-water sump. Depending on your shower needs, a hot water sump with a cold water circulation system could significantly reduce the electricity used for water heating in washing machines.

Additional resources

• National Biodiesel Board
• Diesel and Gas Turbine Publications
• Tecogen, Inc.
• Discount Electric Generator